CODEWITHSUNDEEP
schemesicp
Pseudo-Gorgias
Pseudo-Gorgias

Reputation: 391

SICP 5.2 Assembler

I've been reading through chapter 5 of sicp and have gotten stuck on a piece of code--namely, the assembler presented in 5.2. This is what it looks like:

(define (extract-labels text receive)
    (if (null? text) 
        (receive '() '())
        (extract-labels (cdr text)
        (lambda (insts labels)
            (let ((next-inst (car text)))
                (if (symbol? next-inst)
                    (receive insts
                             (cons (make-label-entry next-inst insts)
                                   labels))
                     (receive (cons (make-instruction next-inst) insts)
                              lables)))))))

Isn't the lambda only going to be called when the text is null? So how can we ask for the car of 'text' then?

EDIT

Thanks for the answers, but I'm still not seeing it. If the text is not null, doesn't extract-lables call itself recursively, until the text is null? In which case how can we call the car of it?

Upvotes: 3

Views: 442

Answers (2)

nneonneo
nneonneo

Reputation: 179422

if takes up to three arguments: a condition, a if-true (then) expression, and an if-false (else) expression. The indentation is all weird, so it looks wrong.

It should appear as

(define (extract-labels text receive)
    (if (null? text) 
        (receive '() '())
        (extract-labels (cdr text)
            (lambda (insts labels)
                (let ((next-inst (car text)))
                    (if (symbol? next-inst)
                        (receive insts
                                 (cons (make-label-entry next-inst insts)
                                       labels))
                        (receive (cons (make-instruction next-inst) insts)
                                  labels)))))))

Observe that the result of the expression is (receive '() '()) if text is null and (extract-labels ...) otherwise.

Upvotes: -1

Femi
Femi

Reputation: 64700

The lambda gets the appropriate scope: at the time the lambda is defined text is not null, so calling car text works fine.

Upvotes: 1

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