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ΓòÉΓòÉΓòÉ 1. Credits ΓòÉΓòÉΓòÉ

                                Common LISP Hints
                               Geoffrey J. Gordon
                              <[e-mail]>
                            Friday, February 5, 1993

                                   Modified by
                                  Bruno Haible
                   <[e-mail]>

About


ΓòÉΓòÉΓòÉ 2. Tutorial ΓòÉΓòÉΓòÉ

Note:  This tutorial introduction to Common Lisp was written for the CMU
environment, so some of the details of running lisp toward the end may differ
from site to site.


ΓòÉΓòÉΓòÉ 2.1. Further Information ΓòÉΓòÉΓòÉ


The best LISP textbook I know of is

  Guy L. Steele Jr. Common LISP: the Language. Digital Press. 1984.

The first edition is easier to read; the second describes a more recent
standard. (The differences between the two standards shouldn't affect casual
programmers.)

A book by Dave Touretsky has also been recommended to me, although I haven't
read it, so I can't say anything about it.


ΓòÉΓòÉΓòÉ 2.2. Symbols ΓòÉΓòÉΓòÉ


A symbol is just a string of characters. There are restrictions on what you can
include in a symbol and what the first character can be, but as long as you
stick to letters, digits, and hyphens, you'll be safe. (Except that if you use
only digits and possibly an initial hyphen, LISP will think you typed an
integer rather than a symbol.) Some examples of symbols:

a
b
c1
foo
bar
baaz-quux-garply

Some things you can do with symbols follow. (Things after a ">" prompt are what
you type to the LISP interpreter, while other things are what the LISP
interpreter prints back to you. The ";" is LISP's comment character: everything
from a ";" to the end of line is ignored.)

> (setq a 5)            ;store a number as the value of a symbol
5
> a                     ;take the value of a symbol
5
> (let ((a 6)) a)       ;bind the value of a symbol temporarily to 6
6
> a                     ;the value returns to 5 once the let is finished
5
> (+ a 6)               ;use the value of a symbol as an argument to a function
11
> b                     ;try to take the value of a symbol which has no value
Error:. Attempt to take the value of the unbound symbol B

There are two special symbols, t and nil. The value of t is defined always to
be t, and the value of nil is defined always to be nil. LISP uses t and nil to
represent true and false. An example of this use is in the if statement,
described more fully later:

> (if t 5 6)
5
> (if nil 5 6)
6
> (if 4 5 6)
5

The last example is odd but correct: nil means false, and anything else means
true. (Unless we have a reason to do otherwise, we use t to mean true, just for
the sake of clarity.)

Symbols like t and nil are called self-evaluating symbols, because they
evaluate to themselves. There is a whole class of self-evaluating symbols
called keywords; any symbol whose name starts with a colon is a keyword. (See
below for some uses for keywords.) Some examples:

>:this-is-a-keyword
:THIS-IS-A-KEYWORD
> :so-is-this
:SO-IS-THIS
> :me-too
:ME-TOO


ΓòÉΓòÉΓòÉ 2.3. Numbers ΓòÉΓòÉΓòÉ


An integer is a string of digits optionally preceded by + or -. A real number
looks like an integer, except that it has a decimal point and optionally can be
written in scientific notation. A rational looks like two integers with a /
between them. LISP supports complex numbers, which are written #c(r i) (where r
is the real part and i is the imaginary part). A number is any of the above.
Here are some numbers:

5
17
-34
+6
3.1415
1.722e-15
#c(1.722e-15 0.75)

The standard arithmetic functions are all available: +, -, *, /, floor,
ceiling, mod, sin, cos, tan, sqrt, exp, expt, and so forth. All of them accept
any kind of number as an argument. +, -, *, and / return a number according to
type contagion: an integer plus a rational is a rational, a rational plus a
real is a real, and a real plus a complex is a complex. Here are some examples:

> (+ 3 3/4)             ;type contagion
15/4
> (exp 1)               ;e
2.7182817
> (exp 3)               ;e*e*e
20.085537
> (expt 3 4.2)          ;exponent with a base other than e
100.90418
> (+ 5 6 7 (* 8 9 10))  ;the fns +-*/ all accept multiple arguments

There is no limit to the absolute value of an integer except the memory size of
your computer. Be warned that computations with bignums (as large integers are
called) can be slow. (So can computations with rationals, especially compared
to the corresponding computations with small integers or floats.)


ΓòÉΓòÉΓòÉ 2.4. Conses ΓòÉΓòÉΓòÉ


A cons is just a two-field record. The fields are called "car" and "cdr", for
historical reasons. (On the first machine where LISP was implemented, there
were two instructions CAR and CDR which stood for "contents of address
register" and "contents of decrement register". Conses were implemented using
these two registers.)

Conses are easy to use:

> (cons 4 5)            ;Allocate a cons. Set the car to 4 and the cdr to 5.
(4 . 5)
> (cons (cons 4 5) 6)
((4 . 5) . 6)
> (car (cons 4 5))
4
> (cdr (cons 4 5))
5


ΓòÉΓòÉΓòÉ 2.5. Lists ΓòÉΓòÉΓòÉ


You can build many structures out of conses. Perhaps the simplest is a linked
list: the car of each cons points to one of the elements of the list, and the
cdr points either to another cons or to nil. You can create such a linked list
with the list fuction:

> (list 4 5 6)
(4 5 6)

Notice that LISP prints linked lists a special way: it omits some of the
periods and parentheses. The rule is: if the cdr of a cons is nil, LISP doesn't
bother to print the period or the nil; and if the cdr of cons A is cons B, then
LISP doesn't bother to print the period for cons A or the parentheses for cons
B. So:

> (cons 4 nil)
(4)
> (cons 4 (cons 5 6))
(4 5 . 6)
> (cons 4 (cons 5 (cons 6 nil)))
(4 5 6)

The last example is exactly equivalent to the call (list 4 5 6). Note that nil
now means the list with no elements: the cdr of (a b), a list with 2 elements,
is (b), a list with 1 element; and the cdr of (b), a list with 1 element, is
nil, which therefore must be a list with no elements.

The car and cdr of nil are defined to be nil.

If you store your list in a variable, you can make it act like a stack:

> (setq a nil)
NIL
> (push 4 a)
(4)
> (push 5 a)
(5 4)
> (pop a)
5
> a
(4)
> (pop a)
4
> (pop a)
NIL
> a
NIL


ΓòÉΓòÉΓòÉ 2.6. Functions ΓòÉΓòÉΓòÉ


You saw one example of a function above. Here are some more:

> (+ 3 4 5 6)                   ;this function takes any number of arguments
18
> (+ (+ 3 4) (+ (+ 4 5) 6))     ;isn't prefix notation fun?
22
> (defun foo (x y) (+ x y 5))   ;defining a function
FOO
> (foo 5 0)                     ;calling a function
10
> (defun fact (x)               ;a recursive function
    (if (> x 0)
      (* x (fact (- x 1)))
      1
  ) )
FACT
> (fact 5)
120
> (defun a (x) (if (= x 0) t (b (- x))))        ;mutually recursive functions
A
> (defun b (x) (if (> x 0) (a (- x 1)) (a (+ x 1))))
B
> (a 5)
T
> (defun bar (x)                ;a function with multiple statements in
    (setq x (* x 3))            ;its body -- it will return the value
    (setq x (/ x 2))            ;returned by its final statement
    (+ x 4)
  )
BAR
> (bar 6)
13

When we defined foo, we gave it two arguments, x and y. Now when we call foo,
we are required to provide exactly two arguments: the first will become the
value of x for the duration of the call to foo, and the second will become the
value of y for the duration of the call. In LISP, most variables are lexically
scoped; that is, if foo calls bar and bar tries to reference x, bar will not
get foo's value for x.

The process of assigning a symbol a value for the duration of some lexical
scope is called binding.

You can specify optional arguments for your functions. Any argument after the
symbol &optional is optional:

> (defun bar (x &optional y) (if y x 0))
BAR
> (defun baaz (&optional (x 3) (z 10)) (+ x z))
BAAZ
> (bar 5)
0
> (bar 5 t)
5
> (baaz 5)
15
> (baaz 5 6)
11
> (baaz)
13

It is legal to call the function bar with either one or two arguments. If it is
called with one argument, x will be bound to the value of that argument and y
will be bound to nil; if it is called with two arguments, x and y will be bound
to the values of the first and second argument, respectively.

The function baaz has two optional arguments. It specifies a default value for
each of them: if the caller specifies only one argument, z will be bound to 10
instead of to nil, and if the caller specifies no arguments, x will be bound to
3 and z to 10.

You can make your function accept any number of arguments by ending its
argument list with an &rest parameter. LISP will collect all arguments not
otherwise accounted for into a list and bind the &rest parameter to that list.
So:

> (defun foo (x &rest y) y)
FOO
> (foo 3)
NIL
> (foo 4 5 6)
(5 6)

Finally, you can give your function another kind of optional argument called a
keyword argument. The caller can give these arguments in any order, because
they're labelled with keywords.

> (defun foo (&key x y) (cons x y))
FOO
> (foo :x 5 :y 3)
(5 . 3)
> (foo :y 3 :x 5)
(5 . 3)
> (foo :y 3)
(NIL . 3)
> (foo)
(NIL)

An &key parameter can have a default value too:

> (defun foo (&key (x 5)) x)
FOO
> (foo :x 7)
7
> (foo)
5


ΓòÉΓòÉΓòÉ 2.7. Printing ΓòÉΓòÉΓòÉ


Some functions can cause output. The simplest one is print, which prints its
argument and then returns it.

> (print 3)
3
3

The first 3 above was printed, the second was returned.

If you want more complicated output, you will need to use format. Here's an
example:

> (format t "An atom: ~S~%and a list: ~S~%and an integer: ~D~%"
          nil (list 5) 6)
An atom: NIL
and a list: (5)
and an integer: 6

The first argument to format is either t, nil, or a stream. T specifies output
to the terminal. Nil means not to print anything but to return a string
containing the output instead. Streams are general places for output to go:
they can specify a file, or the terminal, or another program. This handout will
not describe streams in any further detail.

The second argument is a formatting template, which is a string optionally
containing formatting directives.

All remaining arguments may be referred to by the formatting directives. LISP
will replace the directives with some appropriate characters based on the
arguments to which they refer and then print the resulting string.

Format always returns nil unless its first argument is nil, in which case it
prints nothing and returns a string.

There are three different directives in the above example: ~S, ~D, and ~%. The
first one accepts any LISP object and is replaced by a printed representation
of that object (the same representation which is produced by print). The second
one accepts only integers. The third one doesn't refer to an argument; it is
always replaced by a carriage return.

Another useful directive is ~~, which is replaced by a single ~.

Refer to a LISP manual for (many, many) additional formatting directives.


ΓòÉΓòÉΓòÉ 2.8. Forms and the Top-Level Loop ΓòÉΓòÉΓòÉ


The things which you type to the LISP interpreter are called forms; the LISP
interpreter repeatedly reads a form, evaluates it, and prints the result. This
procedure is called the read-eval-print loop.

Some forms will cause errors. After an error, LISP will put you into the
debugger so you can try to figure out what caused the error. LISP debuggers are
all different; but most will respond to the command "help" or ":help" by giving
some form of help.

In general, a form is either an atom (for example, a symbol, an integer, or a
string) or a list. If the form is an atom, LISP evaluates it immediately.
Symbols evaluate to their value; integers and strings evaluate to themselves.
If the form is a list, LISP treats its first element as the name of a function;
it evaluates the remaining elements recursively, and then calls the function
with the values of the remaining elements as arguments.

For example, if LISP sees the form (+ 3 4), it treats + as the name of a
function. It then evaluates 3 to get 3 and 4 to get 4; finally it calls + with
3 and 4 as the arguments. The + function returns 7, which LISP prints.

The top-level loop provides some other conveniences; one particularly
convenient convenience is the ability to talk about the results of previously
typed forms. LISP always saves its most recent three results; it stores them as
the values of the symbols *, **, and ***. For example:

> 3
3
> 4
4
> 5
5
> ***
3
> ***
4
> ***
5
> **
4
> *
4


ΓòÉΓòÉΓòÉ 2.9. Special forms ΓòÉΓòÉΓòÉ


There are a number of special forms which look like function calls but aren't.
These include control constructs such as if statements and do loops;
assignments like setq, setf, push, and pop; definitions such as defun and
defstruct; and binding constructs such as let. (Not all of these special forms
have been mentioned yet. See below.)

One useful special form is the quote form: quote prevents its argument from
being evaluated. For example:

> (setq a 3)
3
> a
3
> (quote a)
A
> 'a                    ;'a is an abbreviation for (quote a)
A

Another similar special form is the function form: function causes its argument
to be interpreted as a function rather than being evaluated. For example:

> (setq + 3)
3
> +
3
> '+
+
> (function +)
#<Function + @ #x-fbef9de>
> #'+                   ;#'+ is an abbreviation for (function +)
#<Function + @ #x-fbef9de>

The function special form is useful when you want to pass a function as an
argument to another function. See below for some examples of functions which
take functions as arguments.


ΓòÉΓòÉΓòÉ 2.10. Binding ΓòÉΓòÉΓòÉ


Binding is lexically scoped assignment. It happens to the variables in a
function's parameter list whenever the function is called: the formal
parameters are bound to the actual parameters for the duration of the function
call. You can bind variables anywhere in a program with the let special form,
which looks like this:

        (let ((var1 val1)
              (var2 val2)
              ...
             )
          body
        )

Let binds var1 to val1, var2 to val2, and so forth; then it executes the
statements in its body. The body of a let follows exactly the same rules that a
function body does. Some examples:

> (let ((a 3)) (+ a 1))
4
> (let ((a 2)
        (b 3)
        (c 0))
    (setq c (+ a b))
    c
  )
5
> (setq c 4)
4
> (let ((c 5)) c)
5
> c
4

Instead of

(let ((a nil) (b nil)) ...)

you can write

(let (a b) ...).

The val1, val2, etc. inside a let cannot reference the variables var1, var2,
etc. that the let is binding. For example,

> (let ((x 1)
        (y (+ x 1)))
    y
  )
Error: Attempt to take the value of the unbound symbol X

If the symbol x already has a global value, stranger happenings will result:

> (setq x 7)
7
> (let ((x 1)
        (y (+ x 1)))
    y
  )
8

The let* special form is just like let except that it allows values to
reference variables defined earlier in the let*. For example,

> (setq x 7)
7
> (let* ((x 1)
         (y (+ x 1)))
    y
  )
2

 The form

        (let* ((x a)
               (y b))
          ...
        )

is equivalent to

        (let ((x a))
          (let ((y b))
            ...
        ) )


ΓòÉΓòÉΓòÉ 2.11. Dynamic Scoping ΓòÉΓòÉΓòÉ


The let and let* forms provide lexical scoping, which is what you expect if
you're used to programming in C or Pascal. Dynamic scoping is what you get in
BASIC: if you assign a value to a dynamically scoped variable, every mention of
that variable returns that value until you assign another value to the same
variable.

In LISP, dynamically scoped variables are called special variables. You can
declare a special variable with the defvar special form. Here are some examples
of lexically and dynamically scoped variables.

In this example, the function check-regular references a regular (ie, lexically
scoped) variable. Since check-regular is lexically outside of the let which
binds regular, check-regular returns the variable's global value.

> (setq regular 5)
5
> (defun check-regular () regular)
CHECK-REGULAR
> (check-regular)
5
> (let ((regular 6)) (check-regular))
5

In this example, the function check-special references a special (ie,
dynamically scoped) variable. Since the call to check-special is temporally
inside of the let which binds special, check-special returns the variable's
local value.

> (defvar *special* 5)
*SPECIAL*
> (defun check-special () *special*)
CHECK-SPECIAL
> (check-special)
5
> (let ((*special* 6)) (check-special))
6

By convention, the name of a special variable begins and ends with a *. Special
variables are chiefly used as global variables, since programmers usually
expect lexical scoping for local variables and dynamic scoping for global
variables.

For more information on the difference between lexical and dynamic scoping, see
Common LISP: the Language.


ΓòÉΓòÉΓòÉ 2.12. Arrays ΓòÉΓòÉΓòÉ


The function make-array makes an array. The aref function accesses its
elements. All elements of an array are initially set to nil. For example:

> (make-array '(3 3))
#2a((NIL NIL NIL) (NIL NIL NIL) (NIL NIL NIL))
> (aref * 1 1)
NIL
> (make-array 4)        ;1D arrays don't need the extra parens
#(NIL NIL NIL NIL)

Array indices always start at 0.

See below for how to set the elements of an array.


ΓòÉΓòÉΓòÉ 2.13. Strings ΓòÉΓòÉΓòÉ


A string is a sequence of characters between double quotes. LISP represents a
string as a variable-length array of characters. You can write a string which
contains a double quote by preceding the quote with a backslash; a double
backslash stands for a single backslash. For example:

        "abcd" has 4 characters
        "\"" has 1 character
        "\\" has 1 character

Here are some functions for dealing with strings:

> (concatenate 'string "abcd" "efg")
"abcdefg"
> (char "abc" 1)
#\b                     ;LISP writes characters preceded by #\
> (aref "abc" 1)
#\b                     ;remember, strings are really arrays

The concatenate function can actually work with any type of sequence:

> (concatenate 'string '(#\a #\b) '(#\c))
"abc"
> (concatenate 'list "abc" "de")
(#\a #\b #\c #\d #\e)
> (concatenate 'vector '#(3 3 3) '#(3 3 3))
#(3 3 3 3 3 3)


ΓòÉΓòÉΓòÉ 2.14. Structures ΓòÉΓòÉΓòÉ


LISP structures are analogous to C structs or Pascal records. Here is an
example:

> (defstruct foo
    bar
    baaz
    quux
  )
FOO

This example defines a data type called foo which is a structure containing 3
fields. It also defines 4 functions which operate on this data type: make-foo,
foo-bar, foo-baaz, and foo-quux. The first one makes a new object of type foo;
the others access the fields of an object of type foo. Here is how to use these
functions:

> (make-foo)
#s(FOO :BAR NIL :BAAZ NIL :QUUX NIL)
> (make-foo :baaz 3)
#s(FOO :BAR NIL :BAAZ 3 :QUUX NIL)
> (foo-bar *)
NIL
> (foo-baaz **)
3

The make-foo function can take a keyword argument for each of the fields a
structure of type foo can have. The field access functions each take one
argument, a structure of type foo, and return the appropriate field.

See below for how to set the fields of a structure.


ΓòÉΓòÉΓòÉ 2.15. Setf ΓòÉΓòÉΓòÉ


Certain forms in LISP naturally define a memory location. For example, if the
value of x is a structure of type foo, then (foo-bar x) defines the bar field
of the value of x. Or, if the value of y is a one- dimensional array, (aref y
2) defines the third element of y.

The setf special form uses its first argument to define a place in memory,
evaluates its second argument, and stores the resulting value in the resulting
memory location. For example,

> (setq a (make-array 3))
#(NIL NIL NIL)
> (aref a 1)
NIL
> (setf (aref a 1) 3)
3
> a
#(NIL 3 NIL)
> (aref a 1)
3
> (defstruct foo bar)
FOO
> (setq a (make-foo))
#s(FOO :BAR NIL)
> (foo-bar a)
NIL
> (setf (foo-bar a) 3)
3
> a
#s(FOO :BAR 3)
> (foo-bar a)
3

Setf is the only way to set the fields of a structure or the elements of an
array.

Here are some more examples of setf and related functions.

> (setf a (make-array 1))       ;setf on a variable is equivalent to setq
#(NIL)
> (push 5 (aref a 1))           ;push can act like setf
(5)
> (pop (aref a 1))              ;so can pop
5
> (setf (aref a 1) 5)
5
> (incf (aref a 1))             ;incf reads from a place, increments,
6                               ;and writes back
> (aref a 1)
6


ΓòÉΓòÉΓòÉ 2.16. Booleans and Conditionals ΓòÉΓòÉΓòÉ


LISP uses the self-evaluating symbol nil to mean false. Anything other than nil
means true. Unless we have a reason not to, we usually use the self-evaluating
symbol t to stand for true.

LISP provides a standard set of logical functions, for example and, or, and
not. The and and or connectives are short-circuiting: and will not evaluate any
arguments to the right of the first one which evaluates to nil, while or
________ will not evaluate any arguments to the right of the first one which
evaluates to t.

LISP also provides several special forms for conditional execution. The
simplest of these is if. The first argument of if determines whether the second
or third argument will be executed:

> (if t 5 6)
5
> (if nil 5 6)
6
> (if 4 5 6)
5

If you need to put more than one statement in the then or else clause of an if
statement, you can use the progn special form. progn which executes each
statement in its body, then returns the value of the final one.

> (setq a 7)
7
> (setq b 0)
0
> (setq c 5)
5
> (if (> a 5)
    (progn
      (setq a (+ b 7))
      (setq b (+ c 8)))
    (setq b 4)
  )
13

An if statement which lacks either a then or an else clause can be written
using the when or unless special form:

> (when t 3)
3
> (when nil 3)
NIL
> (unless t 3)
NIL
> (unless nil 3)
3

When and unless, unlike if, allow any number of statements in their bodies.
(Eg, (when x a b c) is equivalent to (if x (progn a b c)).)

> (when t
    (setq a 5)
    (+ a 6)
  )
11

More complicated conditionals can be defined using the cond special form, which
is equivalent to an if.. else if ... if construction.

A cond consists of the symbol cond followed by a number of cond clauses, each
of which is a list. The first element of a cond clause is the condition; the
remaining elements (if any) are the action. The cond form finds the first
clause whose condition evaluates to true (ie, doesn't evaluate to nil); it then
executes the corresponding action and returns the resulting value. None of the
remaining conditions are evaluated; nor are any actions except the one
corresponding to the selected condition. For example:

> (setq a 3)
3
> (cond
   ((evenp a) a)        ;if a is even return a
   ((> a 7) (/ a 2))    ;else if a is bigger than 7 return a/2
   ((< a 5) (- a 1))    ;else if a is smaller than 5 return a-1
   (t 17)               ;else return 17
  )
2

If the action in the selected cond clause is missing, cond returns what the
condition evaluated to:

> (cond ((+ 3 4)))
7

Here's a clever little recursive function which uses cond. You might be
interested in trying to prove that it terminates for all integers x at least 1.
(If you succeed, please publish the result.)

> (defun hotpo (x steps)        ;hotpo stands for Half Or Triple Plus One
    (cond
     ((= x 1) steps)
     ((oddp x) (hotpo (+ 1 (* x 3)) (+ 1 steps)))
     (t (hotpo (/ x 2) (+ 1 steps)))
  ) )
A
> (hotpo 7 0)
16

The LISP case statement is like a C switch statement:

> (setq x 'b)
B
> (case x
   (a 5)
   ((d e) 7)
   ((b f) 3)
   (otherwise 9)
  )
3

The otherwise clause at the end means that if x is not a, b, d, e, or f, the
case statement will return 9.


ΓòÉΓòÉΓòÉ 2.17. Iteration ΓòÉΓòÉΓòÉ


The simplest iteration construct in LISP is loop: a loop construct repeatedly
executes its body until it hits a return special form. For example,

> (setq a 4)
4
> (loop
   (setq a (+ a 1))
   (when (> a 7) (return a))
  )
8
> (loop
   (setq a (- a 1))
   (when (< a 3) (return))
  )
NIL

The next simplest is dolist: dolist binds a variable to the elements of a list
in order and stops when it hits the end of the list.

> (dolist (x '(a b c)) (print x))
A
B
C
NIL

Dolist always returns nil. Note that the value of x in the above example was
never nil: the NIL below the C was the value that dolist returned, printed by
the read-eval-print loop.

The most complicated iteration primitive is called do. A do statement looks
like this:

> (do ((x 1 (+ x 1))
       (y 1 (* y 2)))
      ((> x 5) y)
    (print y)
    (print 'working)
  )
1
WORKING
2
WORKING
4
WORKING
8
WORKING
16
WORKING
32

The first part of a do specifies what variables to bind, what their initial
values are, and how to update them. The second part specifies a termination
condition and a return value. The last part is the body. A do form binds its
variables to their initial values like a let, then checks the termination
condition. As long as the condition is false, it executes the body repeatedly;
when the condition becomes true, it returns the value of the return-value form.

The do* form is to do as let* is to let.


ΓòÉΓòÉΓòÉ 2.18. Non-local Exits ΓòÉΓòÉΓòÉ


The return special form mentioned in the section on iteration is an example of
a nonlocal return. Another example is the return-from form, which returns a
value from the surrounding function:

> (defun foo (x)
    (return-from foo 3)
    x
  )
FOO
> (foo 17)
3

Actually, the return-from form can return from any named block -- it's just
that functions are the only blocks which are named by default. You can create a
named block with the block special form:

> (block foo
    (return-from foo 7)
    3
  )
7

The return special form can return from any block named nil. Loops are by
default labelled nil, but you can make your own nil-labelled blocks:

> (block nil
    (return 7)
    3
  )
7

Another form which causes a nonlocal exit is the error form:

> (error "This is an error")
Error: This is an error

The error form applies format to its arguments, then places you in the
debugger.


ΓòÉΓòÉΓòÉ 2.19. Funcall, Apply, and Mapcar ΓòÉΓòÉΓòÉ


Earlier I promised to give some functions which take functions as arguments.
Here they are:

> (funcall #'+ 3 4)
7
> (apply #'+ 3 4 '(3 4))
14
> (mapcar #'not '(t nil t nil t nil))
(NIL T NIL T NIL T)

Funcall calls its first argument on its remaining arguments.

Apply is just like funcall, except that its final argument should be a list;
the elements of that list are treated as if they were additional arguments to a
funcall.

The first argument to mapcar must be a function of one argument; mapcar applies
this function to each element of a list and collects the results in another
list.

Funcall and apply are chiefly useful when their first argument is a variable.
For instance, a search engine could take a heuristic function as a parameter
and use funcall or apply to call that function on a state description. The
sorting functions described later use funcall to call their comparison
functions.

Mapcar, along with nameless functions (see below), can replace many loops.


ΓòÉΓòÉΓòÉ 2.20. Lambda ΓòÉΓòÉΓòÉ


If you just want to create a temporary function and don't want to bother giving
it a name, lambda is what you need.

> #'(lambda (x) (+ x 3))
(LAMBDA (X) (+ X 3))
> (funcall * 5)
8

The combination of lambda and mapcar can replace many loops. For example, the
following two forms are equivalent:

> (do ((x '(1 2 3 4 5) (cdr x))
       (y nil))
      ((null x) (reverse y))
    (push (+ (car x) 2) y)
  )
(3 4 5 6 7)
> (mapcar #'(lambda (x) (+ x 2)) '(1 2 3 4 5))
(3 4 5 6 7)


ΓòÉΓòÉΓòÉ 2.21. Sorting ΓòÉΓòÉΓòÉ


LISP provides two primitives for sorting: sort and stable-sort.

> (sort '(2 1 5 4 6) #'<)
(1 2 4 5 6)
> (sort '(2 1 5 4 6) #'>)
(6 5 4 2 1)

The first argument to sort is a list; the second is a comparison function. The
sort function does not guarantee stability: if there are two elements a and b
such that (and (not (< a b)) (not (< b a))), sort may arrange them in either
order. The stable-sort function is exactly like sort, except that it guarantees
that two equivalent elements appear in the sorted list in the same order that
they appeared in the original list.

Be careful: sort is allowed to destroy its argument, so if the original
sequence is important to you, make a copy with the copy-list or copy-seq
function.


ΓòÉΓòÉΓòÉ 2.22. Equality ΓòÉΓòÉΓòÉ


LISP has many different ideas of equality. Numerical equality is denoted by =.
Two symbols are eq if and only if they are identical. Two copies of the same
list are not eq, but they are equal.

> (eq 'a 'a)
T
> (eq 'a 'b)
NIL
> (= 3 4)
T
> (eq '(a b c) '(a b c))
NIL
> (equal '(a b c) '(a b c))
T
> (eql 'a 'a)
T
> (eql 3 3)
T

The eql predicate is equivalent to eq for symbols and to = for numbers.

The equal predicate is equivalent to eql for symbols and numbers. It is true
for two conses if and only if their cars are equal and their cdrs are equal. It
is true for two structures if and only if the structures are the same type and
their corresponding fields are equal.


ΓòÉΓòÉΓòÉ 2.23. Some Useful List Functions ΓòÉΓòÉΓòÉ


These functions all manipulate lists.

> (append '(1 2 3) '(4 5 6))    ;concatenate lists
(1 2 3 4 5 6)
> (reverse '(1 2 3))            ;reverse the elements of a list
(3 2 1)
> (member 'a '(b d a c))        ;set membership -- returns the first tail
(A C)                           ;whose car is the desired element
> (find 'a '(b d a c))          ;another way to do set membership
A
> (find '(a b) '((a d) (a d e) (a b d e) ()) :test #'subsetp)
(A B D E)                       ;find is more flexible though
> (subsetp '(a b) '(a d e))     ;set containment
NIL
> (intersection '(a b c) '(b))  ;set intersection
(B)
> (union '(a) '(b))             ;set union
(A B)
> (set-difference '(a b) '(a))  ;set difference
(B)

Subsetp, intersection, union, and set-difference all assume that each argument
contains no duplicate elements -- (subsetp '(a a) '(a b b)) is allowed to fail,
for example.

Find, subsetp, intersection, union, and set-difference can all take a :test
keyword argument; by default, they all use eql.


ΓòÉΓòÉΓòÉ 3. Getting Started with Emacs ΓòÉΓòÉΓòÉ


You can use Emacs to edit LISP code: most Emacses are set up to enter LISP mode
automatically when they find a file which ends in .lisp, but if yours isn't,
you can type M-x lisp-mode.

You can run LISP under Emacs, too: make sure that there is a command in your
path called "lisp" which runs your favorite LISP. For example, you could type

        ln -s /usr/local/bin/clisp ~/bin/lisp

Then in Emacs type M-x run-lisp. You can send LISP code to the LISP you just
started, and do all sorts of other cool things; for more information, type C-h
m from any buffer which is in LISP mode.

Actually, you don't even need to make a link. Emacs has a variable called
inferior-lisp-program; so if you add the line

        (setq inferior-lisp-program "/usr/local/bin/clisp")

to your .emacs file, Emacs will know where to find CLISP when you type M-x
run-lisp.


ΓòÉΓòÉΓòÉ <hidden>  ΓòÉΓòÉΓòÉ

This is version 2000.01 of the Common LISP Tutorial.  With the exception of
minor grammatical and typographical corrections, the source text is an exact
copy of the tutorial document created by Bruno Haible (see Credits section at
start of the document).  I have simply compiled the text as a native OS/2
information document (.inf file). I will gladly provide the source code to
anyone wishing to modify, correct, or supplement it for their own use.
Alternatively, I can incorporate electronicly supplied revisions and republish
this document as my time permits.  I can be contacted via eMail to
[e-mail].
Version history:

     15-Jul-2000 . v 2000.01 : Initial compilation