LISP The0ry8e P1‘&.C’61CG
Aconnsen    

Acknowledgements
This book is hmsed on contributions by Mike Gardiner and Charles G
Smith. The Publishers would also like to thank Dr Arthur Norman for
valuable comments on the draft copy.
FIRST EDITION
ISBN 0 907876 Ol 3
Copyright <3 1982, Acornsoft Limited
All Rights Reserved
No part of this book may be reproduced by any means without the prior
permission of the copyright holder. The only exceptions are as
provided for by the Copyright (photocopying) Act or for the purposes
of review or in order for the software herein to be entered into a
computer for the sole use of the owner of this book.
Published by:
Acornsoft Limited
4a Market Hill
Cambridge ·
CB2 3NJ
England

LISP Theory and Practice
l CONTENTS
  1 About LISP 1
2 Using ATOM LISP 3
3 Programming in LISP 5
4 Implementation 21
5 Disc Input/Output 27
6 The Editor 31
7 Further Examples 35
8 Error Handling 43
Glossary 49
Appendix - Memory Map

LISP was invented in the l960's by the American mathematician and
computer scientist John McCarthy, and it has since become one of the
world's major languages for Artificial Intelligence research. It has
also proved to be a valuable tool in fields as diverse as symbolic
logic and medicine.
If you wish to compute payrolls or construct vast mathematical
models of the world's weather systems then LISP is not for you. These
applications conform to the conventional view of computing, that of
ultra—fast arithmetic performed over and over again on fixed-format
data. LISP can do arithmetic but its real strength lies in the ease
with which it can handle flexible structures containing data which may
or may not have anything to do with mathematics. Let us take our
comparison a step further with the following summary:
Conventional Language
Applications:
Finance, Engineering, Applied Maths, Physics.
Data Structures:
Rigid rectangular arrays.
Data Processing:
Fast loops, high precision, floating-point 'number—crunching'.
LISP
Applications:
G Artificial Intelligence, Life-sciences, Database Management,
Pure Maths, ’intelligent' computer games.
F Data Structures: ‘
Flexible structures which may be grown or pruned as
necessary, e.g. lists, trees and graphs.
Data Processing:
List processing, symbol manipulation, recursive functions.
The concept of list processing is fundamental to LISP and in fact
the name of the language is a contraction of this term. As you learn
LISP you will discover just how powerful a programming tool list
processing can be - in fact you may wonder how you ever managed
without it!
Using this Manual
Chapter 3 gives a practical introduction to program ing in LISP. After
working through this, the beginner should be able to follow the
sections on using ATOM LISP, and the descriptions of additional
functions and atoms in the Glossary.
Those who already know LISP should skip the introduction to
programming. Chapter 2 'Using ATOM LISP', and Chapter 4,
'Implementation' give the main details of this implementation, and the
rest of the manual and Glossary can be referred to as and when
necessary.
1

2

I 2 U‘ ATOM usp
2.1 Loading LISP `
ATOM LISP is supplied on cassette and is designed to work on a 8K +
l2K ATOM. To load the system, check the cassette is rewound to the
start, place it in the cassette recorder, and enter:
*RUN"LISP"
followed by RETURN. The ATOM will respond with the message:
PLAY TAPE
whereupon you should set the cassette recorder to play, and then press
* RETURN. 'Snow' will appear on the screen as the LISP interpreter is
loaded into the graphics memory from #8200. The 3K of LISP utilites
and constants supplied on the cassette are then automatically loaded
into memory starting at #2800. The system starts by displaying the
copyright message, followed by the prompt:
”EVALUATE:
LISP is now ready to accept your programs.
To leave LISP, press the BREAK key. The system can be re—entered
without destroying the user's objects or data structures by typing:
LINK#8200
followed by RETURN.
2.2 Saving Sessions
The user can save the whole of the LISP memory area currently in use
(the 'image') on tape. This includes system functions and
data—structures as well as those which have been defined by the user
during the course of the session. The image can be reloaded and the
session continued from the same point. The commands are:
(SAVE ')
(LOAD ')
for example:
(SAVE 'IMAGEl)
The LOAD command can be used only when LISP is already running. If you
wish to perform a 'cold' start you can use the following sequence:
End of Session l.
Enter: » ~
(SAVE 'IMAGEl)
IMAGE is the name chosen for the file in which the memory image will
be saved.
3

Start of Session 2
Enter:
*RUN "LISP"
followed by:
(LOAD 'IMAGEl)
2.3 DOS and COS commands
All the usual commands to the ATOM Disc or Cassette Operating system
can be accessed. This is done from within LISP by using a function
which has the name '*', thus:
(* ')
There must be at least one space between '*' and the command. Notice
also the quote mark in front of the command name. For example:
“EVALUATE: (* 'CAT) h »
This will give a catalogue of the files on disc or cassette.
Abbreviations of DOS commands are not allowed.
4

3 P ‘ ‘ LISP
3.1 Introduction
In this chapter of the manual you will learn LISP the way you learned
your native language — by using it. We shall be following a practical
approach using examples. Each one will teach you some new facts about
LISP and as a bonus you will at the same time be introduced to some
useful programming techniques. _
Every computer has a set of rules telling you what you can and
cannot write. The technical term for such a set of rules is the syntax
of the language. The syntax of LISP is simpler than that of BASIC,
FORTRAN, Algol, COBOL and probably any other commonly used language.
This has several advantages, but there is a price to pay: statements
which look quite different in BASIC, for example, will appear
superficially the same when written in LISP, and this makes them more
difficult to read when you are first learning.
Let's do a brief comparison:
BASIC LISP ·
3 + 7 (PLUS 3 7)
LET x = 1 (SETQ X 1)
PRINT x (PRINT x)
"A B C" (QUOTE (A B C))
PRINT "A B C" (PRINT (QUOTE (A B C)))
x > 5 (GREATERP x 5)
IF x=1 THEN LET Y=2 (coun ((EQ x 1) (SETQ Y 2)))
In each case the LISP function consists of a series of items in
brackets; The first item is always a function name, any remaining
items are known as the arguments of the function - the function acts
on them to produce its result. You will notice that LISP has rather
odd choices for some of its function names. These will be dealt with
later. Most people find all the left and right brackets confusing to
begin with but, don't worry, your eye will quickly become practised at
sorting them out.
3.2 Arithmetic
Load LISP as described in Section 2.l. You should see:
“EVALUATE:
Enter
(PLUS 2 2)
followed by RETURN. LISP will reply as follows:
°VALUE IS: 4
5

Now try adding the following two numbers but leave off the final
parenthesis:
°EVALUATE: (PLUS 7 8 
This time LISP answers by printing a single left-pointing arrow. This
is its way of reminding you that it expects brackets to match up in
pairs. The system will not evaluate the expression until you have
completed it by typing in the missing bracket. To make an expression
clearer, you can type RETURN at any point where a space is allowed.
Enter a single right—parentheSis. LISP will now evaluate the
expression and print the answer:
°VALUE IS: 15
Look at the following expressions and decide what answer LISP will
give to them. Try each one on the ATOM to see if you were right.
a) (PLUS 1 2 3 4)
b) (PLUS 17)
c) (PLUS)
d) (PLUS (PLUS 2 2) (PLUS 2 2) )   ‘
e) (PLUS 5 (PLUS 3 4) 2)
Check that you understand the PLUS function by nmking up some
expressions of your own. If you have problems refer to the definition
of PLUS in the Glossary.
ATOM LISP has five basic arithmetic functions:
PLUS addition
DIFFERENCE subtraction
TIMES multiplication
QUOTIENT integer part of division
REMAINDER remainder of division
They are all used in the same way as PLUS, except that TIMES is the
only other one that can take more than two arguments. Try out some
expressions using them. How would you translate the following into
LISP ?
2 * 4 + 3 * 5 + 7
The answer is 30 - try out your expression to see if it gets it right.
3.3 Lists
We've seen that LISP can do arithmetic but now let's get on to what
the language is really about — list processing. First of all though we
have to know what a list looks like. Here are some examples:
a) (ORANGE APPLE PEAR)
b) (76 43 2l 82)
c) (VALUES 7 23 42)
d) ((A B) (c U) (E P) (G H))
_ 6

Note the similarity between these lists and the arithmetic expressions
we gave LISP. In fact if VALUES was the name of a function, example
(c) could be a valid expression. This illustrates the most important
feature of LISP — program and data are represented and stored in
exactly the same way. The term s-expression (short for symbolic
expression) is used to collectively describe lists and 'atoms', the
other major LISP object, when we are not concerned whether they are
program or data.
( _ The separate items inside the brackets of a list are called the
elements of the list. In example (d) the elements are themselves
lists. In the other examples they are LISP atoms, so called because,
unlike lists, they cannot be further divided. These atoms are of two
kinds: number atoms and character atoms. Number atoms are integer
numbers for use in arithmetic calculations, and character atoms are
used as descriptive data. PLUS is an example of another atom type: is
it a Subr atom (meaning subroutine atom) and it references the
machine—code routine that performs its function.
To tell LISP when a list is to be interpreted as data, rather than
program for evaluation, the quote operator is used. A single quote
before an list or atom turns off evaluation for that item, and LISP
leaves it unaltered for use as data. If you enter any one of the above
example lists, LISP will give an error because they are not legal
programs. A preceeding quote prevents this, for example:
“EVALUATE: '(ORANGE APPLE PEAR)
“VALUE IS: (ORANGE APPLE PEAR)
LISP created an internal representation of the list, and in the
absence of any function using it, has recreated the description from
the structure. Try entering the other lists as data in this way. Put
extra spaces between the items and note that these are removed in
j LISP's printed version. Generally speaking, spaces (and carriage
returns) can be inserted anywhere between items without upsetting
LISP. Also try entering some character atoms as data, noting that
normal character atoms cannot contain spaces. LISP will return them as
it did lists.
3.4 List Processing
List processing consists of picking out items from existing lists and
building up new ones from those items. It is a matter of constructing
and rearranging data structures. Surprisingly it turns out that almost
all the operations we could ever want to perform on lists can be done
using just three simple functions. They are:
` CAR, CDR and CONS
CONS is quite sensibly named in that it is used for constructing
lists. The functions of CAR and CDR just have to be memorised.
Enter the following:
(CAR '(A B C))
not forgetting the quote mark. When you press RETURN you should get
the reply:
VALUE IS: A
7

Now try the following examples:
a) (CAR '(P Q R))
b) (CAR '(A))
C) (CAR ‘((A B) (C D)))
d) (CAR '(PLUS X Y))
Do you see what the function CAR does? It extracts the first element
of the list given as its argument. Next try this:
`EVALUATE: (CDR •(A B c))
You should get:
VALUE Is: (B c)
Can you predict what effect CDR will have on each of the examples
above? Try it on each of them. I; should be clear that CDR takes a
list and returns it with the firs€ element removed.
Enter:
(SETQ L '(A B C D))
LISP will reply with:
”VALUE IS: (A B C D)
SETQ is the equivalent of the BASIC assignment operator '=' : it sets
the value of a variable to the result of an expression. In this case L
is the variable, properly called the 'data object', and (A B C D) is
its new value. The quote has the same effect as before. Try evaluating
L. This should be the result:
CEVALUATE: L
AVALUE IS: (A B C D)
We can now use L instead of (A B C D) in expressions.
Enter the following: °
(CDRIL)
what did you get? What would the answer be if you entered:
(CAR (CDR L))
Try various combinations of CAR and CDR on the list L. See if you can
pick out each individual atom. If you get error messages, look them up
in Chapter 8 and try to find out where you went wrong.
An important point to note is that when working on a list of
atoms, CAR always produces an atom and CDR always pngduces a list. The
exception to this last rule occurs if you remove all the elements from
a list with CDR, for example:
“EVALUATE: (CDR (CDR (CDR (CDR L))))
`VALUE IS: NIL
8

NIL is a special atom that is used to mark the end of lists. It is
equivalent to the 'empty list', the list with no elements:
°EVALUATE: ()
AVALUE IS: NIL
The CONS function constructs lists from atoms and lists. In a
sense, it is the opposite of CAR and CDR because if you take the CAR
and CDR of L you can CONS them back together to make L again, i.e.
AEVALUATE: (CONS (CAR L) (CDR L))
°VALUE is: (A B c D)
Try the following examples using CONS.
a) (CONS 'A '(B C D))
b) (CONS '(A) '(B C D)) T
c) (CONS '(A B) '(C D))
d) (cons ·(A B) '((c D))>
e) (SETQ L '(A B))
(SETQ M '(C D)) A
(CONS (CAR L) M)
f) (CONS (CAR L) (CDR M))
g) (coNs (CAR L) (CDR L))
CONS is then the function which adds items on to the front of an
y existing list.
We made the claim earlier that CAR, CDR and CONS could cope with
most problems of list processing. You might be wondering how this can
be. How, for instance, would you use them to extract the last item
from a list containing a thousand items? In the next few pages we
shall demonstrate how simply LISP can cope with problems such as this
and, in the process, you will discover just how powerful a language it
can be.
One last function we need before we can start writing full
programs is COND, a conditional function. This is LISP's equivalent of
the BASIC IF statement. Here's a comparision:
BASIC LISP
IF X=2 THEN Y=Y+l (COND ((EQ X 2) (SETQ Y (PLUS Y 1)))
COND takes a variable number of arguments of the form:
( )
The 'condition' expression is first evaluated. If the result is NIL,
evaluation carries on with the next expression pair. If the result is
not NIL, the 'result' expression is evaluated. The value returned by
COND is NIL if all of the conditions returned NIL, or the value of the
the result expression that was evaluated. More than one result
expression can be included after a condition expression: they are
evaluated in order and the value returned is that of the last one.
9

3.5 Functions
Enter the following:
(0BL1sT)
' This function provides you with the object list, a list of all the
data objects, including functions, which have so far been defined.
Have a look through the list and you should recognise the ones that we
have used already. To prevent the list from scrolling off the screen
before you have read it, type ESC when it has reached the required
point (ignoring the resulting error message).
Let us invent a new function and place it on the object list.
Suppose we needed a function which would accept any number you gave it
and return that number with one added to it. Thus, if we named the new
function ADD-ONE, we would like it to work as follows:
EVALUATE: (ADD-ONE 23)
VALUE IS: 24
First of all we will write a specification for the function in plain
English and then see how this translates into LISP.
Specification of ADD-ONE in English:
'Define a new function named ADD—ONE that will act on any number that
we give it (let us call it X) to give the result X plus 1'.
Translation:
English LISP
Define a new function (DEFUN...)
named ADD-ONE (DEFUN ADD—ONE...)
that will act on any
number X (DEFUN ADD-ONE (X)...)
to give the result
X plus l (DEFUN ADD—ONE (X) (PLUS X 1))
Type in the definition of ADD—ONE exactly as given in the last line
above. Now look at the object list — your new function name should be
right at the beginning of it. Try the function out:
a) (ADD—0NE 3)
b) (SETQ A 94)
(ADD-ONE A)
c) (ADD-ONE (PLUS A A) )
Have a look at the way the definition has been stored by entering its
name:
“EVALUATE: ADD—ONE
You will notice that it appears slightly different — beginning with
the word 'LAMBDA' -— we will deal with this shortly. Meanwhile try
entering
(SPRINT ADD—ONE)
10

The function SPRINT prints out LISP expressions in a format that makes
them easier to read and understand.
DEFUN is shorthand for SETQ   LAMBDA. We have seen that SETQ
allows you to give an atom a value and that the value may be a number,}
a list or the name of another atom. What we haven't mentioned so far
is that the value may also be a function defintion. To warn the LISP
interpreter to expect a function we use the flag word LAMBDA. Here's
an an example of the use of a LAMBDA definition:
((LAMBDA (x) (PLUS x 1)) 3)
and here is what it means:
LISP English
((LAMBDA (X) ... Apply an anonymous function to an
argument (call it X)
... (PLUS X l)) ... the definition of the function is
'add one to the argument'
... 3) the argument in this case is 3
This of course is just a long-winded way of saying:
(PLUS 3 1)
In a realistic case, the function body would be unlikely to be as
simple as (PLUS X): it is more likely to extend to many lines and
possibly use X more than once. Bearing that in mind, we can see the
usefulness of LAMBDA by using it in conjuction with SETQ, thus:
p (SETQ ADD-ONE '(LAMBDA (X) PLUS X l)))
This creates the data object called ADD—ONE and assigns to it the
LAMBDA expression that follows. At this stage the LAMBDA expression is
treated as an ordinary list. The difference comes when we use ADD-ONE
as though it were a function, i.e. as the first element of list for
evaluation. For example:
"EVALUATE: (ADD—ONE 7 8)
when LISP tries to evaluate this expression, it looks at the first
item expecting it to be a function. In fact, what it finds is the name
of a data object (ADD—ONE). The system does not give up yet however —
it will evaluate anything occupying the function position of an
expression up to twice in an attempt to find a valid function. what it
finds when it evaluates ADD-ONE is a LAMBDA expression and, as we
said, LAMBDA expressions are perfectly good functions in LISP.
At this point, the computer has converted
(AUD-UNE 3)
into
((LAMBDA (X) (PLUS X 1)) 3)
You should recognise this! It is the expression which we dissected at
the beginning of this discussion.
To summarise: ADD-ONE is a data object and not a function. It (or
ll

any other data object we choose) can however serve as a very useful
abbreviation for a function definition in the form of a LAMBDA
expression.
This combination SETQ   LAMBDA is used so frequently that it
itself has an abbreviation, this of course is our friend DEFUN.
Have a look at the following function definitions. What do the
functions do?
a) (DEFUN P2 (X) (TIMES X X))
b) (DEFUN P4 (x) (P2 (P2 x)))
Enter these definitions and then try the functions out. What happens
if you use large numbers as their arguments? Look up the error message
you get in Chapter 8. Notice that function P4 makes use of function P2
so you must have typed that one in first.
Think up some arithmetic functions of your own and enter their
definitions. Do they work exactly as you expected?
Functions can be made to accept several arguments by including the
appropriate number of identifiers in the list after the function name,
like the following:
(DEFUN MYFUNCTION (X Y Z)  )
MYFUNCTION will take three arguments which can then be used in the
body under the names X, Y, and Z.
Now let us attack the problem of producing the last item of a list
of arbitrary length. This is elegantly solved using a function. The
reasoning that follows may seem a little strange at first and you may
need to read the next few paragraphs more than once. Nevertheless if
you can understand them you will have discovered the essence of
programming in LISP. Check the following statements:
Imagine that the function we want already exists and is called LAST,
then:
a) If there were only one item in the list (i.e. if the CDR of the
list is NIL) the answer is the first element of that list. For
example: ‘
(LAST '(P)) should evaluate to P
b) If the list had more than one item then, if you were to take the
CDR of it, its last item would be the same as the last item of the
list itself. For example:
(LAST '(P Q R S)) evaluates to S
and
(LAST '(Q R S)) also evaluates to S
Another way of saying this is:
(LAST '(P Q R S)) is the same as (LAST (CDR '(P Q R S))). —
When you have untangled what the above actually means, it probably
does not look particularly world-shattering, yet amazingly it contains
all the information we need for our definition of the function LAST!
12 l

Let us write it out again more concisely:
If X is a list of only one element:
Then (LAST X) evaluates to (CAR X)
Otherwise (LAST X) evaluates to the same as (LAST (CDR X))
This looks remarkably like a definition. Let us try defining the
function:
English LISP
Define a function
called LAST which
takes A list (DEFUN LAST (X) ...
And returns a value
which, if: ... (COND ...
a) the list contains
only one item ... ((EQ NIL (CDR X)) ...
is the first element
of the list
removed, ... (CAR X))
b) else, ... (T ...
is the result of
applying LAST to the
list with the first
element removed. ... (LAST (CDR X))))
Some of this needs further explanation but first of all let's assemble
the bits to make the complete definition:
(DEFUN LAST (x)
(Coun
((EQ NIL (CDR x)) (CAR x))
( T (LAST (CDR x))> ))
In case you're not convinced that this will work let us type it in
straight away and try it.
Type ix: the definition of LAST exactly as given. Make sure that
you get all the left and right brackets in the proper places. Notice
that the two right brackets on their own at the very end are the ones
which match up with those in front of DEFUN and COND at the beginning
of the function. Notice also that LISP lets you spread a definition
over several lines by keeping count of the brackets. Now try out the
new function:
a) (LAST '(R S T))
b) (LAST •(z)>
c) (LAST '((JIM ALISON) (ARNOLD MARY)))
d) (LAST 56) (
e) (LAST ·A)
r) (LAST (>)
The first three cases Should work and the last three should fail. Why?
Let us take a little time out here to make sure you understand the
definition of LAST. It should be clear that
(EQ NIL (CDR X))
13

detects lists containing only one item because we have already seen
that the end of a list is marked by a NIL in the CDR position, and EQ
returns T (the truth value) if its arguments are equal.
The second part of the COND is:
(T (LAST (CDR X)))
The condition is just T so this result expression will always be
evaluated if none of the preceding conditions turned out to be true.
(LAST (CDR X)) is the really interesting part of the LAST function
and the one that does all the work. It is an example of a recursive
function call.
The extreme in recursive definition might be something like this:
(DEFUN LAST (X)
(LAST X))
LISP would accept this as a definition but when trying to evaluate an
application cnf the function would just go round in circles, always
expecting the answer to appear next time around. However, it would not
go on forever, as would an infinite loop. This is because at each call
of LAST, the system creates a new 'reincarnation' of the function that
is hoping to receive an answer from its successor which it may pass
back to its predecessor. If the level of recursion gets too deep, the
machine will eventually run out of space for storing all the versions
of the function.
If you would like to see just how far LISP will go before running
out of space, try the following:
‘EVALUATE: (DEFUN FOREVER (N)
<(PRINT 'INCARNATION BLANK N)
<(FoREvER (PLUS N 1)))
‘VALUE IS: (LAMBDA (N) (PRINT (QUOTE
INCARNATION) BLANK N) (FOREVER (PLUS N 1)))
‘EVALUATE: (FOREVER 1)
If you wish to investigate along this line further, you may like to
experiment with putting PRINT statements into the following definition
of Ackermann's Function:
(DEFUN ACK
(M N)
(coun
((ZEROP M) (PLUS N 1))
((ZEROP N) (ACK (DIFFERENCE M 1) 1))
(T (ACK (DIFFERENCE M 1) (ACK M (DIFFERENCE N 1))))))
Ackermann's function was originally devised to prove an abstruse point
about the mathematical theory of recursive functions. It is of
theoretical interest because it cannot be defined without recursion.
The mathematical definition of Ackermann's Function is as follows:
Ack [m,n] = n+1 if m=0
Ack [m—l,n] if n=0
Ack [m-l, Ack [m,n-1]] otherwise
14

Since it is doubly recursive you will find that its arguments should
be kept very small if LISP is not going to run out of space, for
example:
AEVALUATE: (ACK 2 3)
We've seen the dangers of circular definitions which cause
infinite recursion. Why then, can we rely on our original version of
LAST working? The answer is that at each incarnation of the function,
the problem has been reduced slightly beause the CDR of a list will
always be shorter than the list itself. Eventually a list containing
only one member will be reached and the incarnation presently
operating will detect it. It will then hand its answer (which in this
case is the answer to the whole problem) back to its predecessor which
will in turn hand it on until it reaches the very first call of the
function. At this point it is handed up one more level to the LISP
system which will print the final answer.
Writing recursive functions is a knack which comes with practice —
here are some hints to get you started.
l Read existing function definitions and try to understand them
thoroughly, (see Chapter 7). Start with simple examples.
2 Experiment with minor modifications to existing functions. Try
inserting PRINT statements at strategic points.
‘ 3 When designing your own recursive functions from scratch, always
start by imagining that the required function already exists and
then use it (but on a smaller argument) within your definition.
4 Make sure that your end conditions (e.g. detecting empty lists)
cover every eventuality.
5 Work through your function by hand as though you were the computer.
A 6 When trying out the function on the computer, start by testing all
the end conditions. Observe that very often you can use very simple
cases, e.g. NIL, as end conditions: many recursive LISP functions
look like this:
(DEFUN XX (Y)
(coun
((EQ Y NIL) )
(T )))
See if you can write a recursive function (called SEPARATE) which
prints out each element of a list on a separate line, for example:
EVALUATE: (SEPARATE •(A E c (D E) F)) ‘
should print
A
B
C S
(D E)
F
A 15

If you get stuck, try using the following guide.
(DEFUN SEPARATE (X)
(coun
((is it a null list?) (If so do nothing))
(otherwise (print first item of list) ( (
(and then SEPARATE the rest of list))))
Now devise a function to reverse a list, thus
°EVALUATE: (REVERSE '(l 2 3 4 5))
‘vALuE IS: (5 4 3 2 1)
If you have trouble approaching the problem, imagine carrying out the
reversal by hand. You might have the items written on cards and laid
out on a tray. The task would be to transfer them one by one to an
empty tray but reversed. The empty tray could be represented in LISP
by the empty list (). Remember that CONS is the function for adding to
lists. Watch out for infinite recursion! Don't forget that (CONS (CAR
X) (CDR X)) is the same as X.
3.6 Property Lists
Before we present a full example LISP application, we'll take a look
at a type of LISP data structure that is an extension of the basic
list concept.
We have already seen that LISP's data objects are more versatile
than the variables in other languages. They can take several different
types of value. In addition each data object has what is called a
property list, whether it has a value or is UNDEFINED. Initially this
is NIL but we can extend it as much or as little as we like. It
contains pairs of items: the first is always the name of the property
(such as hair colour, age, weight etc.) and the second is its value
(e.g. brown, 25, ll stone etc).
Let us see how this works. Enter:
(PUT 'FRED 'HAIRCOLOUR 'BROWN)
This expression will build what is called a dotted—pair which will
look like this:
(HAIRCOLOUR . BROWN)
It will also attach this pair on the property list of FRED.
Now type:
(PUT ’FRED 'AGE '25)
This produces the dotted pair
(AGE . 25)
and adds it to the property list. We can examine the property list of
a data object at any time by using PLIST, for example:
AEVALUATE: (PLIST ’FRED)
°VALUE IS: ((HAIRCOLOUR . BROWN) (AGE . 25))  
16

The real advantage of pmoperty lists, however, is that we can very
easily pick out individual items, and we do this using GET, for
example:
“EVALUATE: (GET 'FRED 'HAIRCOLOUR)
“VALUE IS: BROWN
Note that each property can only have one value, so if we enter:
‘EVALUATE: (PUT 'FRED 'AGE 26)
the value of FRED's property AGE will now be 26.
REMPROP is used to remove a property:
“EVALUATE: (REMPROP 'FRED 'AGE)
“VALUE IS: T
The result is T unless there is no such property, in which case it is
NIL.
3.7 A Real—Life Application
Genealogy is the study of family history. We've all seen what a family
tree looks like and many people find it a fascinating hobby to try and
trace their own origins. LISP is ideal for helping with this sort of
undertaking. Not only is it simple to build up your data structure
using property lists, but in addition it takes very little effort to
write programs which will interrogate the structure and give answers
to questions such as 'Who was X's paternal grandfather' or 'List out
all the first cousins of Y'.
We shall take a fragment of an imaginary family tree as our
example.
FREDERICK
JOHN MARY
PHILIP ANDREW JANE
WILLIAM
Now let us build a segment of this family tree in LISP.
Enter the following sequence. As you go, check that what you are
typing corresponds to information which you can see in the family
tree. Notice also that theme is nothing which represents the lines
joining the names. Quite simply we give each person his or her own
separate property list and note on it certain relationships to other
members of the family.
17

Frederick's details:
(PUT 'FREDERICK 'SON 'JOHN)
(PUT 'FREDERICK 'DAUGHTER 'MARY)
John's details:
(PUT 'JOHN 'SON '(PHILIP ANDREW))
(PUT 'JOHN 'DAUGHTER 'JANE)
(PUT 'JOHN 'FATHER 'FREDERICK)
Jane's details:
(PUT 'JANE 'SON 'WILLIAM)
(PUT 'JANE 'FATHER 'JOHN)
William's details:
(PUT 'WILLIAM 'MOTHER 'JANE)
Notice that we only specify parent/child relationships. This is
sufficient because we can work out the other relationships quite
easily from these. For instance, two people are cousins if they have
the same grandfather and grandmother.
Make sure you typed in the previous portion of the tree correctly
(use PLIST to check this). Now try this:
“EVALUATE: (GET 'JOHN 'DAUGHTER)
if all is well you should get the reply:
‘VALUE IS: JANE
Now 1et's define a function which will tell us the name of the father
of an arbitrary person on the tree. We shall call the function
FATHER-OF.
(DEFUN FATHER-OF (X) (GET X 'FATHER))
Type it in and then try:
a) (FATHER—OF 'JOHN)
b) (FATHER—OF 'JANE)
Check the tree to see if the answers were right. Now we can take
things a stage further. We know that to say 'grandfather of Jane'
means the same as 'father of father of Jane'. This can be translated
directly into LISP. It becomes:
(FATHER—OF (FATHER—OF 'JANE))
So let's define a function GRANDFATHER-OF:
(DEFUN GRANDFATHER—OF (X) (FATHER-OF (FATHER—OF X)))
Type it in and then try it. You should get:
‘EVALUATE: (GRANDFATHER—OF 'JANE)
“VALUE IS: FREDERICK
18

We have of course ignored certain difficulties. The function
GRANDFATHER-OF only works out a person's grandfather on their father's
side. However, we have demonstrated how simple such a function can be.
You might like to work out functions for other relationships. Start
with something simple like GRANDSON and then see if you can work out
the functions SISTER, COUSIN etc. Where there could be more than one
answer, the result should be a list.
Chapter 7 contains a much bigger example application: a program to
find the shortest routes between two towns from a stored
representation of the route map. Have a think about how you would
approach the problem before looking at it. Also included in Chapter 7
are further instructive examples of LISP programs, many of which are
functions that provide the facilities provided as standard on big LISP
systems.
3.8 Conclusion
We hope that this short introduction to LISP programming has given you
some idea of what the language is capable of doing. Unless you are a
computer scientist, you probably still have a few qualms about being
able to tackle large or complicated programs. Remember the maxim 'If
in doubt, try it out!' - LISP is a language where identifying a single
mistake in your logic can result in a huge leap in understanding which
carries over to many other problems. Things really do get easier with
practice and the results are often very worthwhile because LISP
encourages you to think of applications not usually connected with
computing. For example, one medical student used E1 data-structure
representing the way that human bones are connected together as a
revision aid for his examinations, and a puzzle fanatic solved
newspaper problems with the aid of LISP programs. We hope you find
some equally novel uses for the language!
19

20

4 1 1 t t‘
4.1 Atoms
ATOM LISP has four types of atom:
l Number atoms, e.g. 6
-124
31695
2 Character atoms, e.g. CAMBRIDGE
FIELD26
1"#$%&
3 Subr atoms, e.g. PLUS
CAR
PRINT
4 Fsubr atoms, e.g. LOOP
WHILE
COND
4.1.1 Number Atoms
Numbers can have values from -32768 to 32767. Only integers are
available. The evaluation rule for numbers is simple. These ·atoms
always evaluate to themselves. For example, the value of the atom -21
is -21.
4 Numbers between 32767 and -32768 are converted to number atoms.
Numbers outside this range will be returned as unconverted character
atoms.
4.1.2 Character Atoms
Character atoms can have names containing from 0 to 249 characters.
There is no restriction on the characters (within the ASCII character
set). They are used as identifiers ('printnames') of data objects.
Certain character atoms cannot be typed directly because they contain
blanks, full stops, dollar signs or parentheses. lx special. escape
combination is provided for these non—standard atoms. The construction
for the character string XYZ is $$%XYZ%. The two dollar signs are
compulsory. The % characters at each end of the string could have been
any typable character except RETURN. There is no way to type a
character atom which contains RETURN.
4.1.3 Special Character Atoms f
There are four special character atoms: NIL, T, LAMBDA and UNDEFINED.
If the values of any of these are altered then the LISP system could
fail. They should only be used for the special purposes given here.
NIL has two uses. It is used to terminate lists and as the value
'FALSE' in logical expressions. The value of NIL is always NIL and it
cannot have any other properties. F is a synonym for NIL intended for
use in logical expressions.
T represents 'TRUE' in logical expressions. Its value is T. Beware
21

— it is easy to forget that T is special and use it as a variable
name, with frequently disastrous results.
UNDEFINED has the value UNDEFINED. It has a non-standard property
list to save it from removal by the Garbage Collector. It can not have
any normal properties.
LAMBDA is the flag word used to introduce function definitions.
Its value is LAMBDA, but this is not usually very important. It can
have properties.
Data objects may also have properties other than the value. New
properties may be added using PUT and referenced by GET. Undefined
data objects have the value UNDEFINED. This is a rather special state
which means that the object does not appear on the object list. It
will also be liable to be removed from the LISP system by the Garbage
Collector unless it is referenced by some data structure.
4.2 Lists I
Standard lists are formed from.za left bracket '(' followed by the
members of the list followed by a right bracket ')'. The members can
be atoms or can be lists themselves. There is no limit to the number
of members a list can have, other than the amount of computer memory
available.
Before we go on to non-standard lists it is necessary to look at
how LISP stores lists. LISP manipulates data using pointers to the
item rather than the data itself. The pointer is just the address of
the item in memory. Lists are built up from pointer pairs. At a
certain point in a list the first pointer of the pair (CAR) points to
the current member of the list and the second pointer (CDR) to the
next pointer pair in the list. The CDR of the last pointer pair in a
list is specially marked - it is NIL.
If the CDR pointer points to an atom then we have a 'dotted pair'.
This is printed out by LISP as, for example:
(A . B)
Dotted pairs can appear at the end of a list, for example:
(A (LIST) ENDING IN A . PAIR)
4.3 Functions
when LISP is evaluating a list, it assumes that the first atom it
meets after the opening bracket will be a function definition. If not,
it will evaluate anything it finds there up to twice in an attempt to
reach a valid function. For example in
(PLUS x 3)
The character atom PLUS is run; a function definition. However, its
value is a Subr atom which is a function definition. The four types of
function definition are:
l Subr atom
2 Fsubr atom
3 Expr LAMBDA expression
4 Fexpr LAMBDA expression
22

4.3.l Subrs and Fsubrs
Subrs and Fsubrs are LISP system functions written in machine code.
The LISP interpreter always evaluates all the arguments of a Subr. It
then applies the Subr code to these values.
Some special functions have arguments which cannot be evaluated
(e.g. COND) cnr must have their arguments unevaluated (e.g. QUOTE).
These are coded as Fsubrs. The interpreter simply hands them the list
of unevaluated arguments.
It is not possible to create or reference a Subr or Fsubr atom
using E1 READ. Typing FSUBR#4593 for example, would simply create a
character atom.
4.3.2 Exprs and Fexprs
Expr's and Fexpr's are functions written in LISP. Programming in LISP
primarily consists of defining new LISP functions. The way to do this
is to make a LAMBDA expression the value of the name of the function.
A LAMBDA expression has the following syntax:
(LAMBDA parameterdefinition action l action 2 ... )
The action parts of the LAMBDA expression are evaluated one by one and
the last one is returned as the value of the function.
Normally we want to define functions with arguments. In the case
of an Expr the parameter definition is a list of variable names. Here
is a function to give an atom a colour property:
(SETQ PUTCOLOUR
'(LAMBDA (ATM HUE) (PUT ATM 'COLOUR HUE)))
When a PUTCOLOUR expression is evaluated, the arguments will first be
H evaluated one by one. These values are assigned to the corresponding
variable names in the LAMBDA expression. For example, if we evaluate
(PUTCOLOUR 'BANANA 'YELLOW)
then during the evaluation of PUTCOLOUR, ATM has the value BANANA and
HUE has the value YELLOW.
Normally there must be at least as many arguments in the function
call as there are variable names in the LAMBDA expression. However, an
Expr parameter list can also contain optional parameters. These are
indicated by placing the variable names in brackets. Optional
parameters must come after all the simple parameters in the parameter
list. If no argument is provided to match the optional parameter no
error will be signalled; the variable will just take the value NIL.
This default can be changed by turning the optional parameter in
parentheses into a dotted pair. The default value now becomes the CDR
of the dotted pair. Consider this function called PRINT2TO5 :
(LAMBDA (A B (C) (D . 0) (E IS A LIST)) (PRINT A B C D E))
In this example:
A and B are simple parameters.
C is an optional parameter with default value NIL.
D is an optional parameter with default value 0.
E is an optional parameter with default value (IS A LIST).
Optional parameter variables can be used when your program requires
23

variables which are local to a function.
Finally, Fexprs are used where the arguments of the function must
not be evaluated. The parameter description consists of a single
variable name. The unevaluated argument list of the function is
assigned to this variable when the function is called. For example a
function ASSIGN having the same effect as SETQ could be defined in
this way:
(SETQ ASSIGN
'(LAMBDA Q (SET (CAR Q) (EVAL (CADR Q)))))
The Exprs and Fexprs provided by ATOM LISP provide a useful range of
examples of LISP functions. To obtain a listing, type the name of the
Expr or Fexpr in response to the '“EVALUATE:' prompt.
4.4 Garbage collection
During the course of a normal LISP program, data structures are
constantly being added to and trimmed. The computer has to be able to
maintain a pool of data storage locations (called free storage) from
which it can draw. Every so often it becomes necessary to tidy up the
discarded locations and return them in an orderly fashion to free
storage.
The LISP system automatically brings in a program called the
Garbage Collector whenever it detects a lack of working space. The
user can cause a garbage collection at any time by calling on the
function RECLAIM. For example:
“EVALUATE: (RECLAIM)
Programs which are near to the limit of available memory are likely to
be slowed down to a small degree by repeated garbage collections. With
(MESSON 3) the number of reclaimed bytes is shown and so it is also a
good way of finding how much memory is still available.
4.5 Printing
PRINT and PRIN0 print expressions in a standard format:
Character atoms — printed character by character
Number atoms — leading zeros suppressed, no
leading blanks printed
Subr/Fsubr atoms — SUBR# or FSUBR# followed by the
decimal entry address of the
machine code
Lists - elements surrounded by a pair of
brackets and separated by spaces
SPRINT formats lists to make them more readable. The variable
LINEWIDTH contains the number of characters per line assumed by PRINT.
This can be set to suit the output device being used. Its default
value is 31 to suit the ATOM screen.
4.6 System Routines
For completeness, we give a list of all those functions and variables
· 24

which have an effect on the system in addition to their role as
ordinary LISP entities. Details of any not described in this chapter
will be found in the Glossary.
CALL, ERROR, ERRORSET, LINEWIDTH, LOAD, MESSOFF, MESSON, OBLIST, PEEK,
POKE, READ, RECLAIM, SAVE
4.7 Differences from Other LISPs
Atom LISP has no PROG and GO functions. In many other LISPs, PROG has
two logically separate functions:
l Defining local variables
Atom LISP does this in the LAMBDA parameter list. Local
variables are placed in parentheses after the normal function
parameters. See Section 4.3.2.
2 Allowing iterative programs
Atom LISP does this through its LOOP, WHILE, UNTIL
construction.
4.8 Using Extra Memory
Extra RAM can be added to the ATOM, contiguous with the lower text
space, giving more space for the LISP image and stack. On start-up,
the system automatically initialises the stack at the limit of
contiguous RAM at or above #3COO (see the Memory Map in Appendix A).
Thus when you upgrade your ATOM hardware, LISP will automatically make
full use of the extra memory.
25

26

5 D‘ 1 t/o t t
In addition to the LOAD and SAVE commands described in Chapter 2, ATOM
LISP has a set of functions for use with the DOS only.
5.1 OPEN
The OPEN function opens the named file for input or output. Its form
is:
(OPEN  )
If mode is T this indicates that the file already exists. If mode is
NIL then a new file will be created. OPEN returns a value as do all
LISP functions. The value in this case is the 'handle' of the file.
This is az code number which the system allocates temporarily as an
identifier to the file. It is required by all the functions set out
below and so, to avoid having to remember it, you should use OPEN in
conjunction with SETQ. For example,
(SETQ H (OPEN 'DATA NIL))
This opens a new file called DATA and assigns the value of the handle
to H. We can now use H in any function where DATA's handle is
required.
A maximum of five files may be open at a given time although more
can be accessed if necessary by using the CLOSE couunand for those
temporarily not needed.
5.2 CLOSE
The CLOSE function closes El current file iJ1 a tidy way and if
necessary writes an EOF (end—of—file) marker. Its form is:
(CLOSE )
For example:
(CLOSE H)
closes the file whose handle is IL. If the handle is O, all current
files are closed whatever their handles. If a file is closed and then
re-opened, it will be given a new handle. This should be reassigned
using SETQ.
5.3 Input Functions
There are three input functions for use with DOS files:
GETCHAR returns the next character as a character atom
READ returns an entire LISP expression (atom or list)
READLINE returns all the characters upto the next carriage
return as a single character atom
They are all used in the form:
27 0

( )
The handle specifies the file, which must be open when the input
function is used. For example,
‘EVALUATE: (SETQ H (OPEN 'TEXT T))
`VALUE IS: 32
“EVALUATE: (SETQ CHAR (GETCHAR H))
“VALUE IS: A (if A is the first character of TEXT)
Any of these functions can be used without a handle, or with a handle
of 0, to input from the keyboard, for example:
(SETQ EXP (READ 0))
The EOF function detects whether an end—of-file marker has been
reached when reading. Its form is:
(EOF )
It returns T if end—of-file has been reached and NIL if it has not.
5.4 Output Functions
In addition to PRINT, there are two output functions for use with the
DOS. They are:
WRITE0 writes the given items to the specified file
WRITE writes the given items to the specified file,
preceeded by a carriage return
Their form is:
(  )
For example:
(WRITE H 'TEXT 'AREA 'FULL)
A file handle of 0 causes the output to be sent to the screen, so
(WRITE 0 X) is equivalent to (PRINT X) `
and
(WRITEO 0 X) is equivalent to (PRINT0 X)
The READ command requires each record it is reading to be
terminated with a carriage—return character. Thus when using WRITE or
WRITE0 you should finish by using (WRITE ) or (WRITEO 
CR) either of which will send a single carriage—return to the screen.
° 28

Finally, a suitable sequence for printing the contents of file
IMAGEI would be:
(SETQ H (0PEN •IMAGE1 T))
followed by:
(LOOP
(UNTIL (EoF)
(PRIN0 (GETCHAR H)))
(CLOSE H)
29

30

6 Th Ed`t
The LISP editor is itself written as a IJSP function. A listing is
given in the Glossary. Below are summarised the editing commands
followed by an example of their use. The Editor can be used on any
LISP expression but its main purpose is for making changes whilst
developing function definitions.
6.1 Editor Commands
The full set of editor commands is as follows:
Command Function
A Finds CAR of current expression.
B Restores previous current expression (but retaining any
change made).
C CONSes the item which is next typed onto the current
expression.
D Finds CDR of current expression.
R Replaces current expression with the next expression
typed.
X Deletes first item from current expression.
RETURN Prints out current expression (provided another command
is not in effect at the time).
X Where a command takes an expression, a RETURN will be printed and the
expression is then entered, terminated by RETURN. The effect of the
operation is not automatically displayed - the RETURN command can be
used to show this.
An asterisk (*) is printed when a NULL item is reached. It often
indicates that A or D has been used inappropriately. The editor
responds to unrecognised commands with a query (?).
6.2 An Editing example
Let us imagine that we wish to use the editor to change the
expression:
(ONE (TWO 3) 4 XY (5 SIx))
into
(1 (2 3) 4 (5 6) 7)
We shall first of all work through the edit step by step explaining as
we go along. Finally we give an actual listing showing how the edit
would be done in reality.
First set X to the list that requires editing.
AEVALUATE: (SETQ X '(ONE (TWO 3) 4 XY (5 SIX)))
‘VALUE IS: (ONE (TWO 3) 4 XY (5 SIX))
31

In the present rather artificial example this step would not strictly
be necessary but in real life, expressions to be edited will almost
invariably have been assigned to some data object — in this case we
use X.
Now enter:
°EVALUATE: (SETQ TEMP (EDIT X))
This sequence states that we should enter the editor and that it is to
use the value of the atom X as its argument. The expression which
results from the completed editing sequence will be assigned to the
data object TEMP. This is a safety measure; we could have assigned the
result directly back to >< but this loses the original list. In the
event of making a mistake during editing we would not be able to
restore things to their original state.
The Editor will reply to the above like this:
(ONE (TWO 3) 4 XY (5 SIX))
Let us now commence the actual editing process. Enter:
  —
This directs the editor to find the CAR of the current expression,
i.e. ONE. Now enter R  followed by l . This replaces
the current expression, 'ONE', by 'l'. In order to check that this has
been done correctly, we can use B to move back to the full list:
B 
This turns the entire result of the edit so far into the current
expression. You should get the reply:
(1 (Two 3) 4 xy (5 s1x>> .
So far, so good. Now let's change 'TWO' into ‘2'. We can see that TWO
is obtained by taking the CDR, followed by the CAR, followed by the
CAR of the current expression, so enter:
DAA
We typed D before AA because it was necessary to find the CDR before
we could find its CAR.
Enter R  followed by 2 , followed by BB. R 2
replaces 'TWO' by '2'. We typed B twice so as to work our way back
through the nested levels of the argument X.
Now move down to XY by typing DD:
(XY (5 SIX))
Then enter X to delete the first expression XY to give:
((5 s1x>>
We now need to change 'SIX' into '6'. Keep entering D until you
get
((5 SIX))
32

Now we wish to look at the first (and only) item of this list, so
enter:
A
to get (5 SIX). SIX is obtained by taking the CDR followed by the CAR
of this expression so enter:
D followed by A
then enter:
R followed by 6 
All that remains now is to insert the value 7 at the end of the list.
Use B three times to do this. Then enter:
D
to get NIL. Remember that in LISP, NIL is equivalent to the empty list
(). We now need to add our 7 to this and then work back through the
structure. Enter:
C followed by 7
Now use B repeatedly until you get back to the EVALUATE prompt. TEMP
will now hold the edited expression.
Here is the uncommented version of the editing session we have
just worked through. User input is underlined.
AEVALUATE: (SETQ X '(ONE (TWO 3) 4 XY (5 SIX)))
y ‘vALuE Ts: (ONE (Two 3) 4 xy (5 s1x))
°EVALUATE: (SETQ X2 (EDIT X))
(0NE (TWO 3) 4 xy (5 s1x))
A
ENE
B
I
E
(I (TWO 3) 4 XY (5 SIX))
DAA
TWO
B
Z
EE
((2 3) 4 xy (5 s1x))
DD
(XY (5 SIX)) ·
ES
((5 s1x))
ADA
SIX
B
§.
BBB
((5 6))
Q
33

NIL
Q
2.
P2
(4 (5 6) 7)
BBB
TZKLUE xs: (1 (2 3) 4 (5 6) 7)
Note that since the Editor is written in LISP you can use it to edit
itself, putting in extra commands or changing the names of existing
ones to suit yourself. As medium-sized LISP exercises you may like to
try to extend the Editor to provide a command Q (for example) that is
equivalent to B’s in sufficient quantity to get you to the top of the
list being edited, and some form of F command that allows you to find
sublists by matching simple patterns.
/
34

7.1 Nu eric Functions
ABS
Returns the absolute value of a number
(DEFUN ABS (X) (COND ((MINUSP X) (DIFFERENCE 0 X)) (TT X)))
MAX
Returns the largest number in a list
(DEFUN MAX
(x (N . -32767))
(LOOP
(WHILE X N)
(COND ((GREATERP (CAR X) N) (SETQ N (CAR X))))
(SETQ X (CDR X))))
MIN
Returns the smallest number in a list
(DEFUN MIN
(X (N . 32767))
(LOOP
(WHILE X N)
(COND ((LESSP (CAR X) N) (SETQ N (CAR X)))) .
» (SETQ X (CDR X))))
NCR
Returns the number of combinations nCr.
(DEFUN NCR 5
(N K)
(COND
((EQ ZEROP K) 1)
((Eo K N) 1)
(T (PLUS (NCR (DIFFERENCE N l) (DIFFERENCE K 1))
(NCR (DIFFERENCE N 1) K)))))
NCR takes two numeric arguments, N and R, and returns the number of
Combinations of N objects taken R at a time. R must be less than or
equal to N. The recursive definition is as follows:
n .
Cr [n,r] = l if r=0
= l if r=n
= nCr [n—l,r] + nCr [n—l,r-l] otherwise
This is based on Pascal's triangle.
35

PFR
Discovers the prime factors of a number. PFR uses the auxilliary
function PF1.
(DEFUN PFR
(N)
(COND
((zERoP (REMAINDER N 2))
(CONS 2 (PFR (QUOTIENT N 2))))
(T (PF1 N 3))))
(DEFUN PF1
(N M)
(COND
((EQ N 1) NIL)
((LESSP N (TIMES M M)) (LIST N))
((ZEROP (REMAINDER N M)) (CONS M (PF1 (DIVIDE N M) M)))
(T (PF1 N (PLUS M 2)))))
7.2 List Processing Functions
APPEND
Concatenates two lists.
(DEFUN APPEND
(x Y)
(COND ((NULL x) Y) (T (CONS (CAR x) (APPEND (cDR x) Y)))))
Note the difference between APPEND and CONS:
°EVALUATE: (APPEND '(A B C) '(D E F))
°VALUE IS: (A B C D E F)
`EVALUATE: (CONS '(A B C) '(D E F))
`VALUE IS: ((A B C) D E F)
NCONC
Concatenates two lists like APPEND, but does not copy the first list.
(DEFUN NCONC
(x Y (2))
(SETQ Z X)
(COND
((NULL X) Y)
(T (LOOP (WHILE (CDR X) (RPLACD X Y) Z) (SETQ X (CDR X))))))
DELETE
Deletes all occurences of X from the list Y.
(DEFUN DELETE
(X Y)
(COND
((NULL Y) NIL)
((EQ X (CAR Y)) (DELETE X (CDR Y)))
(T (CONS (CAR Y) (DELETE X (CDR Y))))))
36

EQUAL
Tests two lists for equality. NIL is returned if they are equal, and T
otherwise.
(DEFUN EQUAL
(X Y)
(coun
((EQ X Y))
((OR (ATOM X) (ATOM Y)) NIL)
((AND (EQUAL (CAR X) (CAR Y)) (EQUAL (CDR X) (CDR Y))))))
Note that EQUAL tests two lists for equality right down to the
constituent atoms, whereas EQ just tests whether they are in fact the
same list. For example:
“EVALUATE: (SETQ X '(A B C))
‘vALuE IS: (A B C)
”EVALUATE: (SETQ Y '(A B C))
”VALUE IS: (A B C)
”EVALUATE: (EQ X Y)
AVALUE is: NIL
`EVALUATE: (EQUAL X Y)
‘vALUE IS: T -
MEMBER
Test whether atom X is a member of list Y, returning T if so and NIL
otherwise.
(DEFUN MEMBER
(X Y)
(LOOP (WHILE Y) (UNTIL (EQ X (CAR Y)) T) (SETQ-Y (CDR Y))))
For example:
“EVALUATE: (MEMBER 'A '(C D A M))
‘vALUE IS: T
REMOB
Deletes a data object from the object list by removing all its
properties and setting its value to UNDEFINED.
(DEFUN REMOB
(X)
(SETQ X (QUOTE UNDEFINED))
(MAP (QUOTE (LAMBDA (Y) (REMPROP X (CAR Y)))) (PLIST X)))
37

REVERSE
Reverses a list.
(DEFUN REVERSE
(X (2))
(LOOP (WHILE X Z) (SETQ Z (CONS (CAR X) Z)) (SETQ X (CDR X))))
SUBST
Replaces all occurences of Y in list Z to X
(DEFUN SUBST
(X Y Z)
(coun
((EQ Y Z) X)
((ATOM Z) Z)
(T (CONS (SUBST X Y (CAR Z)) (SUBST X Y (CDR Z))))))
MAP
Applies the given function to each element of the list and returns the
result of the last application.
(DEFUN MAP
(FUN LIS)
(coun
((NULL LIS) NIL)
(T (FUN (CAR LIS)) (MAP FUN (CDR LIS)))))
MAPC
Applies a function to the elements of a list like MAP, but returns a
list of the results.
(DEFUN MAPC
(FUN LIS)
(coun
((NULL LIS) NIL)
(T (CONS (FUN (CAR LIS)) (MAPC FUN (CDR LIS))))))
GENSYM
Generates a unique single—character atom for use where new data
objects are required but their names are unimportant.
(DEFUN GENSYM
NIL
(CHARACTER
(SETQ
QGEN
(COND ((NOT (NUMBERP QGEN)) 128) (T (PLUS 1 QGEN))))))
TRACE
Modifies the definition of a fuction so that a message is printed
whenever the function is used, as an aid to debugging. For example,
after
°EVALUATE: (TRACE PLUS)
every use of PLUS will print
38

#(PLUS )
where  is the list of arguments that were supplied to PLUS.
To turn the TRACE facility off, enter
(UNTRACE PLUS)
Note that TRACE should not be used on any of the functions used by
TRACE (for obvious reasonsl), namely:
CAR, CDR, EVAL, PRINT, QUOTE, COND, SET, PUT, GET.
TRACE requires SUBST as defined in this section. The definitions of
TRACE and UNTRACE are as follows:
(DEFUN TRACE
X
(PUT (CAR X) (QUOTE OLD) (EVAL (CAR X)))
(SET
(CAR X)
(SUBST
(CAR X)
(QUOTE ?)
(QUOTE
(LAMBDA
Y
(PRINT (QUOTE #) (CONS (QUOTE ?) Y))
(EVAL (CONS (GET (QUOTE ?) (QUOTE OLD)) Y)))))))
(DEFUN UNTRACE
X
(SET (CAR X) (GET (CAR X) (QUOTE OLD)))
A (REMPROP (CAR x) (QUOTE OLD)))
`VALUE IS: )
7.3 Route Map Example
This example program uses a stored representation of the following map
to determine the shortest route between any two of the towns:
: 15 CAMBRIDGE
BEDFORD
so 20
w#¤TORD ROYSTON
t o
oxFonD 5° Lomoom
39

For each town on the map there is an association list of distances to
the other town that can be reached directly. Thus for WATFORD the
association list is:
((BEDFORD . 30) (RoYsToN . 30) (LONDON . 25) (OXFORD . 40))
This exists as the value of WATFORD's property called NEIGHBOURS. It
is returned by:
°EVALUATE: (GET 'WATFORD 'NEIGHBOURS)
The program and data are supplied on the LISP cassette as the file
IMAGE. This should be loaded as described in Section 2.2. To find the
length of the shortest route between any two of the towns on the map,
use the function DIST with the qouted names of the towns as the
arguments, for example:
°EVALUATE: (DIST 'CAMBRIDGE 'OXFORD)
DISTANCE 85 MILES VIA:
CAMBRIDGE
BEDFORD
WATFRD
OXFORD
°VALUE IS: NIL
The function definitions are as follows:
NPUT
(LAMBDA (A B) (PUT A (QUOTE NEIGHBOURS) B) A)
MEMBER
(LAMBDA
(A B)
(LOOP (WHILE B) (UNTIL (EQ A (CAR B))) (SETQ B (CDR B))))
TREEADD
(LAMBDA
(X TREE)
(COND
( ((NULL TREE) (CONS X (CONS NIL NIL)))
((LESSP (CAR X) (CAAR TREE))
(RPLACA (CDR TREE) (TREEADD X (CADR TREE)))
TREE)
(T (RPLACD (CDR TREE) (TREEADD X (CDDR TREE))) TREE)))
FLATTREE
(LAMBDA
(TREE L)
(COND
((NULL TREE) L)
(T
(FLATTREE
(CADR TREE)
(CONS (CAR TREE) (FLATTREE (CDDR TREE) L))))))
40

NEXT—CITY
(LAMBDA
(FINISH ROUTE-SO—FAR END-ROUTE (NL) (OTHER—ROUTES) (NEW-ROUTE))
(SETQ NL (GET (CADR ROUTE-SO-FAR) (QUOTE NEIGHBOURS)))
(LOOP
(WHILE NL (CONS END-ROUTE OTHER—ROUTES))
(SETQ
NEW—ROUTE
(cons (
(PLUS (CDAR NL) (CAR ROUTE-SO-FAR))
(CONS (cAAB NL) (CDR ROUTE—SO-FAR))))
(COND
((AND
(LESSP (CAR NEW-ROUTE) (CAR END—ROUTE))
(NOT (MEMBER (CAAR NL) (CDR ROUTE—SO-FAR))))
(COND
((EQ (CAAR NL) FINISH) (SETQ END-ROUTE NEW—ROUTE))
(T (SETQ OTHER—ROUTES (CONS NEW—ROUTE
DTBBR-B0uTBs))))))
(SETQ NL (CDR NL))))
SORT
(LAMBDA
(L LIMIT-DISTANCE (TREE))
(LOOP
(WHILE L (FLATTREE TREE NIL))
(COND
((LESSP (CAAR L) LIMIT-DISTANCE)
(SETQ TREE (TREEADD (CAR L) TREE))))
(SETQ L (CDR L))))
NCONC
A (LAMBDA
(x Y (2))
(SETQ Z X)
(COND
((NULL X) Y)
(T (LOOP (WHILE (CDR X) (RPLACD X Y) Z) (SETQ X (CDR X))))))
ADD-TO-PLIST
(LAMBDA
(DIST TOWN1 TOWN2)
(PUT
TOWNI
(QUOTE NEIGHBOURS)
(CONS (CONS TOWN2 DIST) (GET TOWN1 (QUOTE NEIGHBOURS))))
(PUT
TOWN2
(QUOTE NEIGHBOURS)
(CONS (CONS TOWNI DIST) (GET TOWN2 (QUOTE NEIGHBOURS)))))
ROUTE
(LAMBDA
(START FINISH (RSFL) (SHORTEST 32767) (X))
(COND
((EQ START FINISH) (LIST 0 FINISH START))
(T
(SETQ RSFL (LIST (LIST O START)))
(LOOP
(SETQ RSFL (SORT RSFL (CAR SHORTEST)))
41

(WHILE RSFL (COND ((CDR SHORTEST) SHORTEST)))
(SETQ X (NEXT-CITY FINISH (CAR RSFL) SHORTEST))
(SETQ RSFL (NCONC (CDR RSFL) (CDR X)))
(SETQ SHORTEST (CAR x))))))
DIST
(LAMBDA
(Tl T2 (RT))
(SETQ RT (ROUTE T2 T1))
(PRINT (QUOTE DISTANCE) (CAR RT) (QUOTE MILES VIA:))
(LOOP (SETQ RT (CDR RT)) (wH1LE RT) (PRINT (CAR RT))))
42

8 E 1-1 d1`
8.1 Traceback
There are certain things that LISP finds it impossible to do, such as
generating a number larger than 32767 or finding the CAR of an atom.
When one of these is attempted, LISP normally halts and prints some
information about what went wrong. For example, if we try to use XTAB
with a non-numeric argument, we would get the following:
AEVALUATE : (XTAB 'NOTNUMBER)
AERROR NUMBER 21
`ARG: NOTNUMBER
°ARG: (DIFFERENCE S 1)
`ARG: (SETQ S (DIFFERENCE S 1))
AARG: (MINUSP (SETQ S (DIFFERENCE S 1)))
AARG: (UNTIL (MINUSP (sETQ S (DIFFERENCE s 1))))
°ARG: (LOOP (UNTIL (MINUSP (SETQ S (DIFFERENCE S 1))))
(PRIN0 BLANK))
‘S = NOTNUMBER
°ARG: (XTAB 'NOTNUMBER)
"EVALUATE :
The first line of the error message tells us that this error is error
type 21. The various error numbers are described at the end of this
chapter. Error 21 means that the arguments are not all numeric where
this was expected. The remainder of the printout tells us exactly what
was going on when the error occurred. Each line labelled 'ARG:' shows
an expression which LISP is evaluating. These appear in reverse
chronological order: the last ARG is the one that LISP was originally
trying to evaluate and the others are ones that LISP had to evaluate
to do this. In the example we see in the third line
(DIFFERENCE S 1)
It is the expression that actually caused the error as LISP could not
subtract one from the character atom NOTNUMBER. The fourth line shows
that LISP was evaluating
(DIFFERENCE S 1)
as a sub-expression of
(SETQ S (DIFFERENCE S 1))
The following ARGs give progressively higher level expressions.
The top level expression (just before the S= line) is part of the
definition of the XTAB function. When we reach this point at which the
XTAB function was called we have:
`S=NOTNUMBER
This tells us that when the error ocurred the variable S had the value
NOTNUMBER. In general all the local variables of a function would be
printed here. The final ARG shows the cell to XTAB. If XTAB has been
called by another function the printout would have gone on to deal
43

with that.
This summary of events at the time of the error is called the
traceback. You can control whether various parts of it are printed or
not by using the MESSON and MESSOFF functions which are described in
the Glossary. Page mode is automatically set during an error
traceback, so the output will stop and wait for the space bar to be
pressed when it has filled the screen.
8.2 Error—Handling Functions
There are two built—in LISP functions to help in handling errors:
ERROR and ERRORSET. They are described separately in the Glossary.
ERROR prints a message and then triggers the traceback mechanism.
ERRORSET is the more interesting of the two. Normally when an
error occurs, the traceback works through all the currently active
functions and gives a .new* EVALUATE prompt. ERRORSET 'catches' the
traceback on its way past and allows a program to decide for itself
what to do with an error.
8.3 Error numbers
Number Description »
0 No space for stack
1 No space for variable binding
2 No space for new cell
These three error messages mean that LISP has filled the available
workspace and has carried out a garbage collection but could not free
any space. This situation can arise for a number of reasons:
a) The program needs more memory than is available.
b) The program is in a loop, repeatedly allocating more space.
c) The workspace is filled with a large data structure or
program from earlier work .
No traceback is printed because LISP would not be able to find the
workspace needed to print it. Some suggested solutions are:
a) Remove all unnecessary programs and data structures by setting the
relevant variables to UNDEFINED - in particular remove OLDDEF
properties from edited functions.
b) Look through the program for recursive calls which could be
replaced by LOOP - this uses less memory.
c) Reduce the size of the data structures in the program.
d) Check that the logic of the program is correct.
3 Not enough arguments for an Fsubr
This has to be caused by an incorrectly written program. In the
traceback the second ARG will show the offending expression. Check in
the Glossary how many arguments the function should have and edit the
program.
i 4 Interrupt during evaluation
You pressed the ESC key on the keyboard. The traceback will show what
was happening at the time.
44

5 Expression in function position not a function
The first member of a list to be evaluated should be a function
definition. The most common way of obtaining this error is to forget
to define the function. LISP will evaluate a function up to twice to
try to come up with aa Subr atom, Fsubr atom or list beginning with,
LAMBDA.
The first ARG in the traceback shows what LISP had reached when it
gave up trying to produce a function. The second ARG shows the list
being evaluated. The first member of this is the invalid function.
6 Wrong number of arguments for Expr or Subr
There are a number of ways this error can arise:
a) The program gives more than 28 arguments for the function.
b) There are less than the minimum number of arguments required
for the Subr.
c) There are not enough arguments to satisfy all the compulsory
parameters of the Expr.
The second ARG of the traceback shows the offending expression. This
error can easily arise for reason (a) in an APPLY expression.
7 Syntax error in LAMBDA expression
This error can occur either while binding the arguments of an Expr or
Fexpr or while executing it. In either case, check the definition of
the function against the rules in Section 4.3.2. The second ARG of the
traceback will show the incorrectly formed function as the first
member of the list.
y 8 Syntax error in READ
9 Dotted pair syntax error in READ
The s—expression input to a READ function was badly formed. The things
which can go wrong here are:
a) There was an initial full stop or right parentheses
b) A $ in an escape sequence was not immediately followed by
another $.
c) $$ was immediately followed by RETURN.
d) The full stop of a dotted pair was not followed by a single
expression and close bracket.
If you were typing on the keyboard the evidence will still be on the
screen. If the input was from the disc file then the badly formed
expression may take more tracking down. Beware of printing
non—standard atoms onto a disc file. If it was necessary to use the $$
escape sequence to enter the atom it may not read correctly from a
file if it had been printed there in the normal way.
10 String too long
This is aa somewhat unlikely error which could happen during a READ.
LISP strings can contain up to 249 characters. If an atom longer than
this is typed this error will arise.
45

11 Interrupt during PRINT
You have pressed the ESC key while LISP was printing an expression. If
LISP starts to print a circular list it is useful to be able to stop
the printout. There is no traceback — it would only have started to
print the same list again!
12 Attempt to print unknown atom type
This error message really means that LISP has crashed. It will only
occur if:
a) The LISP system or a LISP program has loaded badly
b) You have altered the value of a special atom like T
13 Syntax error in COND expression
This is a programming error - a COND expression has been written
incorrectly. This error arises when there is an atom where a
condition/action list was expected. The first ARG will show the atom
and the second ARG the incorrect COND expression. Check the expression
against the syntax given in the Glossary.
14 Attempt to take CAR or CDR of an atom
CAR and CDR and their compounds can only be applied to lists or dotted
pairs. The first ARG of the traceback will show the offending atom. In
a compound function like CADR this may be an intermediate result
rather than the original argument. The solution is to review the logic
of the program to emsure that CAR and CDR are never applied to an
atom.
15 LISP ERROR function called
The ERROR function generates this error number and corresponding
traceback. The message printed by ERROR will presumably give some clue
as to what went wrong.
16 Attempt to assign a value to other than a data object
Only data objects can be assigned values. If a SET or SETQ function
has any other type of object as its first argument, this error will
arise. The second ARG in the traceback shows the expression in which
this occurred.
If this happens in a SETQ expression it is a programming error:
make sure the first argument is the character atom name of E1 data
object. If it occurs in a SET expression the logic of the program may
be at fault.
17 Not character atom where expected
This error can have a variety of causes. One of the arguments (usually
the first) of an expression should have evaluated to a character atom,
but did not.
The second ARG of the traceback gives the expression in which the
error arose.
46 `

18 _ Arithmetic overflow
An arithmetic expression has produced a result greater than 32767 or
less than -32768, or there was an attempt to divide by zero. The first
ARG of the traceback normally gives the value of the last argument of
the offending expression. The expression is shown in the second ARG.
19 First argument of RPLACA or RPLACD not a list
The first argument of RPLACA and RPLACD must be a list or dotted pair.
The first ARG of the traceback gives the value of the second argument
of the function. The second ARG gives the RPLACA or RPLACD expression.
20 Property list syntax error
This error occurs when a property list does not have the following
form:
((propname . property) (propname . property) ... )
The error usually means that the non-standard property list of
UNDEFINED has been accessed or the property list has been corrupted by
the use of RPLACA or RPLACD at some point.
21 Arguments not all numeric
Certain functions expect all their arguments to be number atoms. If
any argument is not, this error occurs. The first ARG shows the value
of the last argument of the incorrect expression. The expression
itself is shown in the second ARG.
22 Function in APPLY not Expr or Subr
The first argument of APPLY must evaluate to a Subr atom or an Expr
list. This condition is stricter than that for an expression in the
function position of a list and it is easy to make an error.
23 Bad argument list for APPLY
The second argument of APPLY must be a list. This error occurs when it
is not. The first ARG gives the incorrect second argument. The second
ARG shows the expression in which it occurred.
24 For future expansion
25 For future expansion
47

4 8  

This reference section lists all the atoms available initially on the
LISP object (OBLIST). More can be added by using DEFUN or SETQ to
define new variables. For each atom an explanation of its value and
use is given. In the case of functions, the explanation includes
whether it is a Subr, Fsubr, Expr or Fexpr and how the function works.
All Subrs and Exprs evaluate all their arguments before applying the
function. This is not repeated in each explanation. Fsubrs and Fexprs
are always fully explained.
Character atoms
carriage return
space
$
( (
)
These are single character atoms which are difficult to type into
LISP expressions. Each one has itself as its value. They can be
accessed from the following atoms:
Atom Value
CR return
BLANK space (
DOLLAR $ (
LPAR (
. RPAR )
PERIOD .
For example:
(PRINT DOLLAR 35)
will print
$35
AND Fsubr
(AND predicate predicate ... )
AND can have any number of arguments. It returns T if and only if
the values of all its arguments are non NIL. Otherwise it returns
NIL. AND does not necessarily evaluate all its arguments. It goes
through the list evaluating them one by one until:
A a) the value of an argument is NIL — the value returned is then
NIL.
b) the end of the list is reached — the value returned is T.
For example:
(AND 'TEA 'CAKES)
f 49

has the value T
(AND (LISTP X) (CDR X))
is T if X is a list of at least one member. In this example (CDR
X) is never evaluated if X is not a list.
APPLY Subr
(APPLY function argumentlist)
As usual for a Subr, APPLY evaluates both its arguments. The first
must evaluate to a Subr atom or to an Expr list. The second must
evaluate to a list of arguments for the function. The function is
then applied to the arguments and the value of this is returned.
The difference between this and a normal Subr or Expr application
is that the arguments are not evaluated again before the function
is applied. For example:
(APPLY cons ·(A B))
is equivalent to
(CONS 'A 'B)
and has the value
(A. B)
Another example is:
(APPLY '(LAMBDA (X Y) (SET X Y))
'(A B))
The expression returns the value B, and A will be set to have the
value B. The LAMBDA list must have a quote because the first
argument of APPLY is always evaluated.
Note that
(APPLY SET '(A B))
would cause an error because SET is an Fsubr, not a Subr or Expr.
ATOM Subr
(ATOM argument)
ATOM returns G? Lf its argument is any atom, character, number,
Subr or Fsubr, If the argument is a list or dotted pair ATOM
returns NIL. For example:
(ATOM 'CHARATOM)
has value T.
(ATOM CAR)
has the value T because CAR is a Subr atom.
(ATOM '(TAWNY SNOWY))
50 _

has value NIL.
BLANK Character atom
The initial value of BLANK is a single space character. It is
particularly useful in separating items in PRINT functions.
CALL Subr
(CALL address accumulator)
CALL provides a method of using machine code subroutines from
LISP. The first argument is numeric and supplies the address of
the subroutine. If the address is larger than 32767 the equivalent
two's—complement negative number must be used. The second argument
defines the contents of the microprocessor's accumulator on entry
to the routine and is optional. This argument must also be
numeric. The low byte of the number is loaded into the
accumulator. CALL returns a number giving the contents of the
accumulator when the routine returns. For example:
(CALL -29)
calls za monitor routine which waits until a key is pressed and
then puts the result in the accumulator.
CAR CDR Subr
CAAR CADR
CDAR CDDR I
(CxxR list)
These functions return a part of the list obtained by evaluating
their argument. It is an error for the argument to be an atom. CAR
picks out the first member of a list or the first member of a
dotted pair. CDR returns the remainder of the list with the first
member removed. Applied to a dotted pair, CDR returns the second
member. For example:
(CAR '(FORD LEYLAND DATSUN))
has value FORD.
(CDR '(FORD LEYLAND DATSUN))
has value (LEYLAND DATSUN).
(CAR '(DRIVE . LEFT))
has value DRIVE.
CAAR etc. provide for compound uses of CAR and CDR. For example:
(CADR X)
is equivalent to
(CAR (CDR X))
51

CHARACTER Subr
(CHARACTER number)
CHARACTER returns a single character. The ASCII Code for the
character is given by the number which is the value of the
argument. This is useful for generating characters not available
on the keyboard. For example:
(CHARACTER 65)
has the value A
(CHARACTER 7)
is the BELL character
CHARCOUNT Expr
(CHARCOUNT number item)
Returns the value of the first argument minus the number of
characters required to print 'item', with numbers assumed to
require 6 Characters. CHARCOUNT is used by SPRINT. Its definition
is as follows:
(LAMBDA
(X LEFT)
(COND
((ATOM X)
(COND
((GREATERP
LEFT (CHARS X)) (DIFFERENCE LEFT (CHARS X)))))
(T
(LOOP
(UNTIL
(ATOM X)
(CHARCOUNT
X
(DIFFERENCE LEFT (COND (X 4) (T —2)))))
(WHILE
(SETQ
LEFT
(CHARCouNT (CAR x) (DIFFERENCE LEFT 1))))
(sETQ X (CDR x))))))
CHARP Subr
(CHARP expression)
CHARP returns T if its argument is a character atom. Otherwise it
returns NIL. For example:
(CHARP A)
has value T
(CHARP 7)
has value NIL.
52 i

CHARS Subr
(CHARS characteratom)
Returns the number of characters in the character atom given. For
example:
(CHARS 'HELLO)
has the value 5.
CLOSE Subr
(cLosE )
Closes a current file and if necessary writes an EOF (end-of—file)
marker. If the handle is 0, all current files are closed whatever
their handles.
COND Fsubr
(COND (condition action action ...)
(condition action action ...) ...)
COND is the main structure provided in LISP for testing and acting
on conditions. COND can have any number of arguments. These
arguments are treated in a special way.
The LISP interpreter looks at the arguments of COND one by one
in the order in which they appear. Each argument is a list. The
value of the first member of the list - the condition - determines
the action taken. If the value is NIL, the rest of that list is
ignored and LISP goes on to the next argument. If the value is T
or anything other than NIL, the remaining members of the list are
evaluated one by one. The value of the last member is returned as
the value of the COND expression. There is no limit on the number
of members of the list.
There are two special cases:
a) All of the conditions evaluate to NIL. In this case the
value of COND is NIL.
b) An argument list has no actions, only a condition. Here the
value of the condition is returned if it is not NIL.
For example:
(conn
((EQ X 3) (PRINT 'THREE) 'YES)
(T 'NO))
This will print THREE and return the value YES if the value of X
is 3. Otherwise it will return the value NO. COND expressions very
often finish with a T condition to cope with all remaining
possibilities.
Another example is:
(coun
((OR (MINUSP HOUR) (GREATERP HOUR 23))
(ERRoR '(HouR Now TIME OF nAY)))
((LESSP HOUR 7) 'NIGHT)
((LESSP HOUR l2) 'MORNING)
((LESSP HOUR 18) 'AFTERNOON)
((LESSP HOUR 22) 'EVENING)
53

(T 'NIGHT))
This returns the time of day depending on the value of HOUR. There
is an error message if HOUR does not represent a valid time.
CONS Subr
(CONS argl arg2)
CONS returns its two arguments combined as the dotted pairz;
‘ (argl . arg2)
Usually arg2 will be a list, in which case this will have the
effect of adding argl to the beginning of the list. CONS is the
reverse of CAR and CDR. For example:
(CONS (CAR X) (CDR X))
has the same value as X:
(CONS 'HOVERCRAFT '(SHIP AEROPLANE))
has the value
(HOVERCRAFT SHIP AEROPLANE)
CONS can be used to create dotted pairs:
(CONS 'X 'Y)
has the value (X.Y).
CR Character atom
The value of CR is a character atom of one character.
The character is a return, hexadecimal SOD
DEFUN Fexpr
(DEFUN functionname parameterlist action action ... )
DEFUN is a convenient method of defining functions. None of the
arguments are evaluated. This use of DEFUN is exactly equivalent
to:
(SETQ functionname
'(LAMBDA parameterlist action action .... ))
The value returned by DEFUN is the function name. 'functionname'
is the name of the function you wish to define. 'parameterlist' is
the list of arguments and local variables which the function uses
(see Section 4.3.2). Any number of actions can be given for the
function to carry out. For example:
We wish to define the function ADD2 which adds two to a number.
(DEFUN ADD2 (x) (PLUS x 2))
ADD2 would then have the value
54

(LAMBDA (X) (PLUS X 2))
and (ADD2 36)
would have the value 38 as required.
DIFFERENCE Subr
(DIFFERENCE numberl number2)
The value returned is numberl minus number2. For example, if A has
the value l6 and B has the value 3 then :
(DIFFERENCE A B)
has value l3.
DOLLAR Character atom
The value of DOLLAR is the character atom $. It is useful to use
DOLLAR rather than the dollar sign in programs to avoid confusing
the READ routines which expect $ to introduce an escape sequence.
EDIT Expr
(EDIT expression) _
Details of EDIT together with an example of its use are given in
Chapter 6. Its definition is as follows: _
(LAMBDA
(A (Q))
(LOOP
(SETQ Q (PRINO (GETCHAR)))
(UNTIL (EQ Q 'B) A)
(SETQ
A
(coun
((EQ Q 'R) (PRINT) (READ))
((EQ Q CR) (PRINO A) (PRINT) A)
((EQ Q 'C) (PRINT) (cons (READ) A))
((ATOM A) (PRINO (QUOTE *)) A)
((EQ Q 'D) (CONS (CAR A) (EDIT (CDR A))))
((EQ Q ’A) (CONS (EDIT (CAR A)) (CDR A)))
((EQ Q ·x) (CDR A))
(T (PRINO '?) A)))))
EOF Subr
(EOF )
Detects whether an end—of-file marker has been reached when
reading. It returns T if end—of-file has been reached and NIL if
it has not.
EQ Subr
(EQ argl arg2)
EQ will return T if one of the following conditions applies to the
value of the two arguments:
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a) They are the same character atom.
b) They are equal numbers.
c) They are identical lists in LISP memory.
Otherwise EQ returns NIL. For example:
(EQ 4 4)
has value T.
(EQ 'FRED 'FRED)
has value T.
(EQ ·FRE¤ 4)
has value NIL.
For a pair of lists to be equal they must share the same
memory as well as being identical in form. Usually EQUAL (see
Chapter 7) is more useful on lists.
ERROR Subr
(ERROR arg arg ...)
ERROR behaves like PRINT in that it prints a return followed by
the value of each of its arguments. Having done that it generates
error number l5 and the usual error traceback occurs. Here is an
example of its use in checking that L is a list before finding its
CDR.
(COND ((ATOM L) (ERROR L BLANK 'NOTLIST))
(T (cnn 1.)))
ERRORSET Fsubr
(ERRORSET expression)
Normally when aux error accurs in evaluating an expression, the
traceback works through all the function calls and halts the
program. ERRORSET is a means of preventing this and keeping
control in the program. 'expression' is an s-expression in which
you think that an error could occur. The possible outcomes are as
follows:
a) If there is no error, ERRORSET acts like LIST, i.e.
(ERRORSET expression)
is equivalent to
(LIST expression)
b) If an error occurs then the value is a numeric atom giving
the error number.
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ATOM or LISTP therefore provide an easy way of determining whether
an error has occured. The following would be a typical use of
ERRORSET:
(LOOP (ssmo X (ERRORSET (READ)))
(UNTIL (LISTP X) (CAR X))
(PRINT '(TRY TYPING THAT AGAIN)) )
EVAL Subr
(EVAL arg)
EVAL returns its argument evaluated one extra time. As EVAL is a
Subr, the total effect is that arg is evaluated twice. For
example:
(SETQ CRITERION 'COLOUR)
(SETQ COLOUR 'GREEN)
After this
(EVAL CRITERION)
has the value GREEN.
F Special character atom
The initial value of F is NIL. It is intended that F is used for
the logical value 'false‘ just as T is used for 'true'.
FSUBRP Subr
(FSUBRP arg)
FSUBRP tests whether its argument is a Fsubr atom. If so it
returns T, if not it returns NIL. For example:
(FsuBRP conn)
has value T.
(FSUBRP ‘COND)
has value NIL because COND is a character atom, even though its
value is an Fsubr atom.
GET Subr
(GET characteratom propertyname)
GET searches the property list of the character atom for the given
property name. If the name is found, the property is returned. If
there is no such property, the value of GET is NIL. For example:
If ALBERT has properties HEIGHT 172
EYES BLUE
CONVICTIONS NIL
(GET 'ALBERT 'HEIGHT)
has value l72.
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(GET ·ALBERT ·HA1Rc0LoUR)
has value NIL.
(GET 'ALBERT 'CONVICTIONS)
has value NIL.
Note that it is not possible to distinguish between an absent
property and a NIL property using GET.
GREATERP Subr
(GREATERP numberl number2)
GREATERP returns T if numberl is greater than number2. Otherwise
the value is NIL. For example:
(GREATERP 4 3)
has value T.
GETCHAR Subr
(GETCHAR)
GETCHAR returns a single—character character atom. This character
is the next byte to be input via the current input channel.
Normally GETCHAR will return the next key to be pressed on the
ATOM keyboard.
LAMBDA Special character atom
(LAMBDA parameterdefinition action action ... )
LAMBDA is used as the first member of Expr and Fexpr function
definitions. The full syntax of function definitions is discussed
in Chapter 4. LAMBDA has the value LAMBDA.
LESSP Subr
(LESSP numberl number2)
LESSP returns T if numberl is numerically less than number2.
Otherwise it returns NIL. Both arguments must be numeric. For
example:
(rnssp 5 10)
has value T.
(rnssp 6 6)
has value NIL.
LINEWIDTH Nu ber
LINEWIDTH is used by SPRINT as the approximate limit on the number
of characters printed per line. For example:
(SETQ LINEWIDTH 31)
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will set the correct line width for the screen. For use with a
printer, values of 78 or 130 might be more appropriate.
LIST Subr
(LIST argl arg2 ... )
List can have up to 28 arguments. After evaluating them all, as
normal for a Subr, it makes up a list with the values as the
members. This list is returned. If there are no arguments the
value is NIL. For example:
(LIST 'A 3 '(X Y))
has value (A 3 (X Y)).
LISTP Subr
(LISTP arg)
LISTP returns T if the argument is a list or a dotted pair, and
NIL if the argument is an atom. It is the opposite of ATOM in
that:
(NULL (ATOM X))
is equivalent to
(LISTP X)
For example:
(LISTP (CONS 'A X))
has value T.
(LISTP 3)
has value NIL.
LOAD Subr
(LOAD filename)
This command is described in Chapter 2.
For example:
(LOAD 'BASE)
loads the disc file named BASE.
LOOP Fsubr
(LOOP argl arg2 ... )
LOOP can have any number of arguments. LOOP repeatedly evaluates
all its arguments, starting each time with argl and working in
order through the list. A LOOP will never terminate unless one of
the arguments contains a WHILE or UNTIL function. When the WHILE
or UNTIL function is satisfied the LOOP will halt just before it
evaluates its next argument. The value returned is the value of
the last argument evaluated, i.e. the one containing the satisfied
59

WHILE or UNTIL. The following will print
X IS n
for values of n from 6 to O and then return DONE:
(SETQ X 7)
(LOOP (UNTIL (MINUSP (SETQ X (SUB l X)))
'DONE)
(PRINT ’X BLANK 'IS BLANK X) )
LPAR Character atom
LPAR has the character atom '(' as its value.
MESSON Subr
MESSOFF
(MESSON messagenumber)
(MESSOFF messagenumber)
MESSON and MESSOFF are used to control whether certain system
messages are printed. MESSON will allow the message to be printed
and MESSOFF will suppress it. Once the status of a message has
been set this way it remains unchanged until a disastrous error
occurs or another MESSON or MESSOFF expression is evaluated. The
message numbers are as follows:
Number Message
l Garbage collection bytes collected
2 Garbage collection number
4 Error number
8 Error top level ARG'S ‘
l6 Error traceback
l28 Read depth prompt
The value returned is the message number. For example:
(Masson 16)
will cause error tracebacks to be printed in future.
(MEssoFF 16)
will prevent error tracebacks from being printed. Note that
message numbers can be added together, so for example:
(MEss0FF 3)
will turn off both garbabge collecter messages simultaneously.
MINUSP Subr
(MINUSP argument)
MINUSP returns the value T if its argument is a negative number
atom. Otherwise the value is NIL. For example:
(MINUSP -4)
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has value T.
(MINUSP ·FREU)
has value NIL.
NIL Special atom
NIL is a special character atom. It always has NIL as its value.
It is used to terminate lists and as the false value in logical
expressions.
NOT Subr
NULL
(NOT argument)
(NULL argument)
NOT and NULL behave in exactly the same way. They return the value
T if the argument is NIL. Otherwise they return NIL. For example:
(NOT T)
has value NIL.
(NULL *(I 2 3))
has value NIL.
(NULL F)
has value T. The value of F is NIL.
NUMBERP Subr
(NUMBERP argument)
NUMBERP returns T if its argument is a number atom. Otherwise the
value is NIL. For example:
(NUMBERP (PLUS 6 5))
has value T.
(NUMBERP 'LETTERS)
has value NIL.
OBLIST Subr
(OBLIST)
OBLIST returns a list of all the character atoms known to LISP,
except those that:
a) have the value UNDEFINEDVALUE
b) and have no properties
These conditions eliminate those atoms which are being used as
character strings rather than as atoms with an interesting value.
The order of the atoms in the list is the order in which they
6l

appear in the ATOM memory, highest addresses coming first.
OPEN Subr
(OPEN  )
The OPEN function opens the named file for input or output. If
mode is T this indicates that the file already exists. If mode is
NIL then a new file will be created. OPEN returns the file handle.
OR Fsubr
(OR predicate predicate ...)
OR returns NIL only if the values of all its arguments are NIL.
Otherwise it returns T. OR evaluates its arguments one by one from
the left hand end until it finds one whose value is not NIL. OR
then returns T, without evaluating the remaining arguments. If OR
reaches the end cu? the argument list without finding a non—NIL
value it returns NIL. For example:
(OR 'X P Q)
has value T and P and Q do not get evaluated.
(OR NIL NIL)
has value NIL.
(OR)
has value NIL.
ORDINAL Subr g
(ORDINAL characteratom)
ORDINAL returns the numeric ASCII code for the first character in
the character atom. If the character has zero length the number
zero is returned. It is an error for the argument not to be a
character atom. For example:
(ORDINAL 'ALPHABET)
has value 65.
PEEK Subr
(PEEK address)
PEEK returns a number which represents the contents of the memory
location whose address is given in the argument. The address must
be a number atom. Addresses above 32767 can be represented by
their equivalent two's-complement negative number. For example:
(PoKE 768 21)
will place 2l in location 768 (see POKE).
(PEEK 768)
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will then have the value 2l.
PERIOD Character atom
PERIOD is a character atom consisting of one full stop character.
PLIST Subr
(PLIST characteratom)
PLIST returns the property list of the character atom. If the atom
has no properties it returns NIL. The property list is a list of
dotted pairs of the form:
((propname . property) (propname . property) ...)
For example:
if JACQUELINE has no properties to begin with:
(PUT 'JACQUELINE 'AGE 22)
(PUT ’JACQUELINE 'FLOWER ’TULIP))
Then
(PLIST 'JACQUELINE)
has the value
((FLOWER . TULIP) (AGE . 22))
PLUS Subr
(PLUS number number ...)
PLUS returns the sum of all its arguments, which must all be
number atoms. PLUS can have any number of arguments up to 28. For
example:
(PLUS 6 2 -3)
has value 5.
(PLUS)
has value 0.
POKE Subr
(POKE address number)
POKE stores the single byte representation of its second argument
in the memory location specified by the first argument. Both
arguments must be numbers. For example:
(POKE 34 TOP)
PRIN0 Subr
(PRINO argl arg2 ...)
PRIN0 prints its arguments one by one, starting from the current
63

printing position. Because PRINO is a Subr, all the arguments are
evaluated before printing starts.
N.B. No spaces or other separators will be placed between the
arguments - this must be done explicitly using BLANK or CR. PRINO
returns the value of its last argument. For example: .
(PRINO 'PRICE BLANK DOLLAR 32)
would print
PRICE $32 p
and has value 32.
PRINT Subr
(PRINT argl arg2 ...)
PRINT is exactly like PRINO except that printing begins on a new
line.
PROGN Fsubr
(PROGN expression expresion ...)
PROGN evaluates its arguments one by one and returns the value of
its last argument. It is useful in allowing several expressions to
be evaluated where the syntax would otherwise only allow for one.
For example:
if X has the value 5
(PRocN (PRINT x DOLLAR) (LIST x)>
would print 5$ and have the value (5).
PUT Subr
(PUT characteratom propertyname property)
PUT places the property on the property list of the character atom
referenced by the property name. The value of the PUT expression
is the property, the third argument. An atom never has two
properties with the same name: a second PUT with the same property
name will replace the old property. PUT is well illustrated by
this function to save the present value of an atom on its property
list as the OLDVALUE property:
(LAMBDA (X) I
(PUT x ·oLnvALuE (EVAL x)))
The value could then be restored by;
(SETQ GULL (GET 'GULL 'OLDVALUE))
QUOTE Fsubr
(QUOTE argument)
QUOTE returns its argument unevaluated. It is equivalent in effect
to the single quote mark. For example: ,
64

(QUOTE APPLES)
has value APPLES.
'PEARS
has value PEARS.
QUOTIENT Subr
(QUOTIENT dividend divisor)
QUOTIENT returns the integer part which results from dividing the
dividend by the divisor. Both arguments must be numbers. For
example:
(QUOTIENT 5 2)
has value 2.
(QUOTIENT -400 6)
has value -66.
READ Subr
(READ)
READ reads one s—expression from the current input stream (e.g.
the keyboard or disc file) and returns it unevaluated. No prompt
is given.
READLINE Subr
(READLINE )
Returns all the characters upto the next carriage return from the
file whose handle is given as a single character atom.
RECLAIM Subr
(RECLAIM)
RECLAIM causes a garbage collection. The value returned is always
NIL. With the garbage collection messages on, this is a useful way
of finding out how much free memory is left.
REMAINDER Subr
(REMAINDER dividend divisor)
REMAINDER returns the remainder from the division of the dividend
by the divisor. This value is always positive. Both arguments must
be numbers. For example:
(REMAINDER 5 2)
has value l.
(REMAINDER -400 66)
has value 4.
y 65

REMPROP Subr
(REMPROP characteratom propertyname)
REMPROP will remove a property from the property list of an atom.
The value is T if there was originally such a property on the list
and NIL if there was not. For example, after
(PUT 'EAGLE 'ISBIRD T)
the value of
(REMPROP 'EAGLE 'ISBIRD) p
is T the first time that it is evaluated and NIL afterwards.
RPAR Character atom
RPAR is a single character atom consisting of a close bracket.
RPLACA Subr
(RPLACA list arg2)
RPLACA returns the list with its CAR replaced by the second
argument. The first argument must be a list or dotted pair. There
are no restrictions on the second argument. RPLACA should be used
with caution as it actually alters the list cell in memory.
Therefore other data structures using the same cell will also be
changed. For example:
(RPLACA '(A B C) 'FIRST)
has value
(FIRST B C)
If the above use of RPLACA was part of a program, execution would
alter the actual expression to
(RPLACA '(FIRST B C) 'FIRST)
RPLACD Subr
(RPLACD list arg2)
RPLACD returns the list with its CDR replaced by the second
argument. It is therefore similar to RPLACA and the same cautions
apply. For example:
(RPLACD '(A B C) 'FIRST)
has value (A . FIRST) _
SAVE Subr
This function is described in Section 2.2. For example:
(SAVE 'BASE)
saves the workspace as the disc file BASE.
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SET Fsubr
(SET characteratom newvalue)
SET evaluates both its arguments. The first must evaluate to a
character atom. The value of this character atom is set to be the
value of the second argument. This value is returned. For example:
(SET (QUOTE X) (PLUS 2 3))
has value 5 and X now has the value 5.
SETQ Fsubr
(SETQ characteratom newvalue)
SETQ evaluates only its second argument. The first argument must
be a character atom. The value of the second argument becomes the
value of this character atom, and is the value returned. For
example:
(SETQ x (PLUS 25 -20))
has value 5 and X now has the value 5
SUBRP Subr
(SUBRP arg)
SUBRP tests whether its argument is a Subr atom. If so it returns
T, otherwise NIL. For example:
(SUBRP SUBRP)
has value T.
(SUBRP COND)
has value NIL.
SPRINT Expr
(SPRINT arg columnindent)
SPRINT prints its first argument with the list structure neatly
displayed. The column indent is optional. If supplied it must be
zero or a positive number. Printing is offset by the column
indent. SPRINT uses XTAB and CHARCOUNT as subroutines.
(LAMBDA
(x (N . 0))
(conn
((OR (ATOM X) (CHARCOUNT X (DIFFERENCE LINEWIDTH N)))
(PRINO X))
(T
(PRINO LPAR)
(SPRINT (CAR X) N)
(SETQ N (PLUS N 3))
(LOOP
(SETQ X (CDR X))
(COND ((AND X (ATOM X)) (PRINO PERIOD X)))
(UNTIL (ATOM X) (PRINO RPAR))
67

(XTAB N)
(SPRINT (CAR X) N)))))
T Character atom
T has the value T. It is used as the 'true' logical value. For
example:
(NUMBERP 43)
has value T.
TIMES Subr
(TIMES number number ... )
TIMES returns the product of all its arguments. The arguments must
all be numeric. There can be any number of arguments up to 28. For
example:
(TIMES 28 4 -2)
has value -224.
UNDEFINED Special character atom
UNDEFINED has the value UNDEFINED. When a new character atom is
created by READ, GETCHAR or CHARACTER it is given this initial
value. Atoms with the value UNDEFINED and no properties are
special in that:
a) they do not appear on the OBLIST
b) they can be removed by the garbage collector if they are
not accessed by any data structure
UNTIL Fsubr
(UNTIL predicate action action .... )
UNTIL is used in conjunction with LOOP q.v. The predicate is
evaluated. If its value is NIL, UNTIL returns NIL. If the value is
not NIL, the following changes occur:
a) the actions are evaluated one by one and the last one is
returned as the value of UNTIL. If there are no actions the
value of the predicate is returned.
b) a flag is set to terminate the first surrounding LOOP. This
termination does not occur until LOOP attempts to evaluate
its next top level argument.
See LOOP for an example.
WHILE Fsubr C
(WHILE predicate action action ... )
WHILE is used in conjunction with LOOP q.v. It is similar to
UNTIL, except that the predicate must be NIL for the actions to be
evaluated and the LOOP to terminate. If WHILE has no actions when
the predicate is NIL, NIL is returned.
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WRITE Subr
(WRITE handle iteml item2 ... )
Writes the given items to the file whose handle is given,
preceeded by a carriage return.
WRITE0 Subr
(WRITE0 handle iteml item2 ... )
Writes the given items to the file whose handle is given.
XTAB Expr
(XTAB number)
Prints the specified number of spaces at the start of a new line.
XTAB is used by SPRINT. Its definition is as follows:
(LAMBDA
(S)
(PRINT)
(Loop
(UNTIL (MINUSP (SETQ S (DIFFERENCE S l))))
" (PRIN0 BLANK)))
ZEROP Subr
(znnop arg)
ZEROP returns T if the argument is the number zero. Otherwise it
returns NIL. For example:
(zERoP (REMAINDER 8 2))
has value T.
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70

Append1x
Mem0ryMap
Contents Address
/////////////
9800
LISP
INTERPRETER
8200
SCREEN
. 8000
//////////////
/////////////
////////////// 3c00
W STACK
IMAGE
2800
//////////////

. FIRST EDITION
  Copyright © 1982 Acornsoft Limited
ISBN 0 907876 Ol 3 `7