The apply function in SICP/Scheme

Question

I've asked a few questions here about Scheme/SICP, and quite frequently the answers involve using the apply procedure, which I haven't seen in SICP, and in the book's Index, it only lists it one time, and it turns out to be a footnote.

Some examples of usage are basically every answer to this question: Going from Curry-0, 1, 2, to ...n.

I am interested in how apply works, and I wonder if some examples are available. How could the apply procedure be re-written into another function, such as rewriting map like this?

#lang sicp

(define (map func sequence)
    (if (null? sequence) nil
        (cons (func (car sequence)) (map func (cdr sequence)))))

It seems maybe it just does a function call with the first argument? Something like:

(apply list '(1 2 3 4 5)) ; --> (list 1 2 3 4 5)
(apply + '(1 2 3))        ; --> (+ 1 2 3)

So maybe something similar to this in Python?

>>> args=[1,2,3]
>>> func='max'
>>> getattr(__builtins__, func)(*args)
3

@Barmar yes but it's also an option in DrRacket: https://gyazo.com/28836fa26506a63c1c9d5dbb8ec9a386 and https://docs.racket-lang.org/sicp-manual/index.html — David542, Jun 17 '21 at 21:02

Barmar · Answer 1 · 2021-06-17T21:13:36.037

2

apply is used when you want to call a function with a dynamic number of arguments.

Your map function only allows you to call functions that take exactly one argument. You can use apply to map functions with different numbers of arguments, using a variable number of lists.

(define (map func . sequences)
  (if (null? (car sequences))
      '()
      (cons (apply func (map car sequences))
            (apply map func (map cdr sequences)))))

(map + '(1 2 3) '(4 5 6))
;; Output: (5 7 9)

edited Jun 17 '21 at 21:13

answered Jun 17 '21 at 21:01

Barmar

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thanks, should that say `map` though instead of apply? (I get an error when trying to do `apply +`.) – David542 Jun 17 '21 at 21:05
1

Yes, sorry about that. – Barmar Jun 17 '21 at 21:13

Will Ness · Accepted Answer · 2021-06-19T07:22:22.633

You asked to see how apply could be coded, not how it can be used.

It can be coded as

#lang sicp

; (define (appl f xs)    ; #lang racket
;   (eval 
;     (cons f (map (lambda (x) (list 'quote x)) xs))))

(define (appl f xs)      ; #lang r5rs, sicp
  (eval
    (cons f (map (lambda (x) (list 'quote x)) 
                 xs))
    (null-environment 5)))

Trying it out in Racket under #lang sicp:

> (display (appl list '(1 2 3 4 5)))
(1 2 3 4 5)

> (display (     list   1 2 3 4 5 ))
(1 2 3 4 5)

> (appl + (list (+ 1 2) 3))
6

> (     +       (+ 1 2) 3 )
6

> (display (appl map (cons list '((1 2 3)  (10 20 30)))))
((1 10) (2 20) (3 30))

> (display (     map       list  '(1 2 3) '(10 20 30)  ))
((1 10) (2 20) (3 30))

Here's the link to the docs about eval.

It requires an environment as the second argument, so we supply it with (null-environment 5) which just returns an empty environment, it looks like it. We don't actually need any environment here, as the evaluation of the arguments has already been done at that point.

thanks, that `5` is pretty odd in the procedure call, but it seems that's just the version (if so, why can't the implementation just figure that out itself?) Also, do you want to add a link to the answer for the `null-environment` as well? — David542, Jun 18 '21 at 16:42
it's the same link. :) just scroll down few lines, it's literally the next entry. :) — Will Ness, Jun 18 '21 at 16:52

The apply function in SICP/Scheme

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