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3. Using Guile to program in Scheme

In this section I give a tutorial introduction to programming in Scheme, with a slant toward the interesting things that can be done in Guile.

This section will try to touch on many of the interesting and cool aspects of Guile, showing you how new types of problems can be solved with Guile. Note that using Guile as a library with libguile.a is described in its own chapter (see section 4. Guile in a Library). Also note that some small examples are given in 1. Jump Start.

To get started you need to know how to program in Scheme (a dialect of LISP). Fortunately Scheme is a small, clean language and is not hard to learn. It is also used in many undergraduate courses to introduce computer programming.

I will not try to teach you Scheme here (although you might end up learning by example), since there are many good books on the subject, listed in 7. Where to find more Guile/Scheme resources. (1)


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3.0.1 Hello World

Our first program is the typical Scheme "hello world" program. Put the following code in a file called hello.scm (this can be find in `examples/scheme/hello.scm').

 
#!/usr/local/bin/guile -s
!#

(display "hello world")
(newline)

Then run guile on it. One way to do so is to start up guile and load this file:

 
<shell-prompt> guile
guile> (load "hello")

Another way is to make the file executable and execute it directly. Notice how Guile recognizes a -s option which tells it to run a script and then exit. Guile also has a new type of block comment enclosed by #! and !#, so that you can make executable Scheme scripts with the standard UNIX #! mechanism.

In the given example, the first line is used to invoke the Guile interpreter (make sure you correct the path if you installed Guile in something other than /usr/local/bin). Once Guile is invoked on this file, it will understand that the first line is a comment. The comment is then terminated with !# on the second line so as to not interfere with the execution mechanism.


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3.0.2 A bunch of operations in Scheme

Here is some code you can type at the guile> prompt to see some of the Scheme data types at work (mostly lists and vectors). I have inserted brief comments before each line of code explaining what happens.

 
;; make a list and bind it to the symbol ls
guile> (define ls (list 1 2 3 4 5 6 7))
       =>
;; display the list
guile> ls
       => (1 2 3 4 5 6 7)
;; ask if ls is a vector; #f means it is not
guile> (vector? ls)
       => #f
;; ask if ls is a list; #t means it is
guile> (list? ls)
       => #t
;; ask for the length of ls
guile> (length ls)
       => 7
;; pick out the first element of the list
guile> (car ls)
       => 1
;; pick the rest of the list without the first element
guile> (cdr ls)
       => (2 3 4 5 6 7)
;; this should pick out the 3rd element of the list
guile> (car (cdr (cdr ls)))
       => 3
;; a shorthand for doing the same thing
guile> (caddr ls)
       => 3
;; append the given list onto ls, print the result
;; NOTE: the original list ls is not modified
guile> (append ls (list 8 9 10))
       => (1 2 3 4 5 6 7 8 9 10)
guile> (reverse ls)
       => (10 9 8 7 6 5 4 3 2 1)
;; ask if 12 is in the list --- it obviously is not
guile> (memq 12 ls)
       => #f
;; ask if 4 is in the list --- returns the list from 4 on.
;; Notice that the result will behave as true in conditionals
guile> (memq 4 ls)
       => (4 5 6 7)
;; an if statement using the aforementioned result
guile> (if (memq 4 ls)
           (display "hey, it's true!\n")
           (display "dude, it's false\n"))
       @print{hey, it's true!}
       =>
guile> (if (memq 12 ls)
           (display "hey, it's true!\n")
           (display "dude, it's false\n"))
       @print{dude, it's false}
       =>
guile> (memq 4 (reverse ls))
       => (4 3 2 1)
;; make a smaller list ls2 to work with
guile> (define ls2 (list 2 3 4))
;; make a list in which the function sin has been
;; applied to all elements of ls2
guile> (map sin ls2)
       => (0.909297426825682 0.141120008059867 -0.756802495307928)
;; make a list in which the squaring function has been
;; applied to all elements of ls
guile> (map (lambda (n) (expt n n)) ls)
       => (1 4 27 256 3125 46656 823543)

 
;; make a vector and bind it to the symbol v
guile> (define v #(1 2 3 4 5 6 7))
guile> v
       => #(1 2 3 4 5 6 7)
guile> (vector? v)
       => #t
guile> (list? v)
       => #f
guile> (vector-length v)
       => 7
;; vector-ref allows you to pick out elements by index
guile> (vector-ref v 2)
       => 3
;; play around with the vector: make it into a list, reverse
;; the list, go back to a vector and take the second element
guile> (vector-ref (list->vector (reverse (vector->list v))) 2)
       => 5
;; this demonstrates that the entries in a vector do not have
;; to be of uniform type
guile> (vector-set! v 4 "hi there")
       => "hi there"
guile> v
       => #(1 2 3 4 "hi there" 6 7)


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3.0.3 Using recursion to process lists

Here are some typical examples of using recursion to process a list.

 
;; this is a rather trivial way of reversing a list
(define (my-reverse l)
  (if (null? l)
      l
      (append (my-reverse (cdr l)) (list (car l)))))
(my-reverse '(27 32 33 40))
=> (40 33 32 27)


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3.0.4 Processing matrices

Suppose you have a matrix represented as a list of lists:

 
(define m
  (list
   (list 7 2 1 3 2 8 5 3 6)
   (list 4 1 1 1 3 8 9 8 1)
   (list 5 5 4 8 1 8 2 2 4)))

Then you could apply a certain function to each element of the matrix in the following manner:
 
;; apply the function func to the matrix m element-by-element;
;; return a matrix with the result.
(define (process-matrix m func)
  (map (lambda (l)
         (map func l))
       m))
Notice that I have used the Scheme map procedure because I am interested in the matrix that results from the application of func, rather than in the side effects associated with applying func.

This could be invoked with (process-matrix m sin) or (process-matrix m (lambda (x) (* x x))); for example:

 
(process-matrix m (lambda (x) (* x x)))
=> ((49 4 1 9 4 64 25 9 36) (16 1 1 1 9 64 81 64 1) (25 25 16 64 1 64 4 4 16))

To print a representation of the matrix, we could define a generalized routine:
 
;; proc is a procedure to represent the single element,
;; row-proc is a procedure that is invoked after each row.
;; Example: proc could be (lambda (x) (begin (display x) (display " ")))
;; and row-proc could be (lambda (l) (display "\n"))
(define (represent-matrix m proc row-proc)
  (for-each (lambda (l)
              (begin
                (for-each proc l)
                (row-proc l)))
            m))

And then invoke it with
 
(represent-matrix m
                  (lambda (x) (begin (display x) (display " ")))
                  (lambda (l) (begin (display "\n"))))
@print{7 2 1 3 2 8 5 3 6}
@print{4 1 1 1 3 8 9 8 1}
@print{5 5 4 8 1 8 2 2 4}

Now we write a helper routine that uses Scheme closures to make objects with state that then receive messages to draw little squares.

But let us take it one step at a time. I will start by showing you a simple example of object in Scheme. The object I make here represents a cell, which could be a cell in a matrix. The cell responds to commands to draw itself, to return the next cell, and so forth. Guile does not currently have a Tk interface, so I will leave the hooks for graphical rendering. In a future release of Guile I will add graphical rendering messages to the cell object.

 
;; cell-object.scm: routines for creating and manipulating cell objects

;; (the-x, the-y) is the initial position of the cell.
;; the-color is a string representing a color; must be something Tk can grok.
;; square-size is the size of the square that gets drawn.
;; (sizex, sizey) is the size of the matrix.
(define (MAKE-CELL the-x the-y the-color square-size sizex sizey)
  (define (get-x) the-x)
  (define (get-y) the-y)

  (define (set-x! new-x)
    (set! the-x new-x)
    the-x)
  (define (set-y! new-y)
    (set! the-y new-y)
    the-y)
  (define (get-color) the-color)
  (define (set-color! new-color)
    (set! the-color new-color)
    the-color)
  (define (next!)
    (set! the-x (+ the-x 1))
    (if (>= the-x sizex)
	(begin
	  (set! the-x 0)
	  (set! the-y (+ the-y 1))))
	(if (>= the-y sizey)
	    (begin
	      (display "CELL next!: value of y is too big; not changing it\n")
	      (set! the-y (- the-y 1))))
	(cons the-x the-y))
  (define (draw)
    (let* ((x0 (* the-x square-size))
	   (y0 (* the-y square-size))
	   (x1 (+ x0 square-size))
	   (y1 (+ y0 square-size)))
      (display "I should draw a ")
      (display the-color)
      (display " rectangle with corners at ")
      (display x0) (display y0) (display x1) (display y1)
      ))

  ;; self is the dispatch procedure
  (define (self message)
    (case message
      ((x)            get-x)
      ((y)            get-y)
      ((set-x!)       set-x!)
      ((set-y!)       set-y!)
      ((color)        get-color)
      ((set-color!)   set-color!)
      ((next!)        next!)
      ((draw)         draw)
      (else (error "CELL: Unknown message -> " message))))
  ;; and now return the dispatch procedure
  self
  )

What does this procedure do? It returns another procedure (self) which receives a message (x, y, set-x!, set-y!, ...) and takes an action to return or modify its state. The state consists of the values of variables the-x, the-y, the-color and so forth.

Here are some examples of how to use MAKE-CELL and the cell object it creates:
 
(define c (MAKE-CELL 0 0 "red" 10 7 9))

;; retrieve the x and y coordinates
((c 'x))
=> 0
((c 'y))
=> 0
;; change the x coordinate
((c 'set-x!) 5)
=> 5
((c 'x))
=> 5
;; change the color
((c 'color))
=> "red"
((c 'set-color!) "green")
=> "green"
((c 'color))
=> "green"
;; now use the next! message to move to the next cell
((c 'next!))
=> (6 . 0)
((c 'x))
=> 6
((c 'y))
=> 0
;; now make things wrap around
((c 'next!))
=> (0 . 1)
((c 'next!))
=> (1 . 1)
((c 'next!))
=> (2 . 1)
((c 'x))
=> 2
((c 'y))
=> 1

You will notice that expressions like (c 'next) return procedures that do the job, so we have to use extra parentheses to make the job happen. This syntax is rather awkward; one way around it is to define a send procedure:

 
;; send makes object syntax a bit easier; instead of saying
;;     ((my-cell 'set-x!) 4)
;; you can say
;;     (send my-cell 'set-x! 4)
(define (send obj . args)
  (let ((first-eval (apply obj (list (car args)))))
    (if (null? (cdr args))
	(first-eval)
	(apply first-eval (cdr args)))))

You can see that send passes the message to the object, making sure that things are evaluated the proper number of times. You can now type:

 
(define c2 (MAKE-CELL 0 0 "red" 10 7 9))
(send c2 'x)
=> 0
(send c2 'set-x! 5)
=> 5
(send c2 'color)
=> "red"
(send c2 'set-color! "green")
=> "green"
(send c2 'next!)
=> (1 . 0)
(send c2 'x)
=> 1
(send c2 'y)
=> 0

This is the simplest way of implementing objects in Scheme, but it does not really allow for full object-oriented programming (for example, there is no inheritance). But it is useful for object-based programming.

Guile comes with a couple more complete object-oriented extensions to Scheme: these are part of slib (see section `Object' in SLIB: the portable Scheme library and see section `Yasos' in SLIB: the portable Scheme library).


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This document was generated by Ingo Ruhnke on September, 12 2002 using texi2html