CODEWITHSUNDEEP
scheme
user8149311
user8149311

Reputation:

Don't understand how this let-let-lambda works

I'm back with another scheme question. This is an example from Dybvig's book The Scheme Programming Language, and I understand what is happening:

(let ([x 'a])
  (let ([f (lambda (y) (list x y))])
    (f 'b))) --> (a b)

However he immediately follows it up with a modified example that I don't understand:

(let ([f (let ([x 'sam])
             (lambda (y z) (list x y z)))])
  (f 'i 'am)) --> (sam i am)

In the call (f 'i 'am) I don't know what's going on, that's my confusion. I don't see how f is defined with two parameters. How do the lambda parameters bubble up, for lack of a better term, to f?? I would think that the code let ([x 'sam]) would block that from happening. But that isn't the case obviously. I hope my English phrasing of my confusion makes sense.

Upvotes: 2

Views: 40

Answers (1)

wizzwizz4
wizzwizz4

Reputation: 6426

f is defined as (lambda (y z) (list x y z)), where x is defined as 'sam.

Let's break this down a bit.

(let ([x 'sam])
    (lambda (y z) (list x y z)))

This evaluates to the lambda (which I'll call that_lambda because I can).

(let ([f that_lambda])
  (f 'i 'am)) --> (sam i am)

This evaluates to (that_lambda 'i 'am).

Because that_lambda was defined* in the scope* where x was 'sam, x used inside that_lambda refers to 'sam.

* It's so long since I've done Lisp that I don't know whether my terms are correct.

Upvotes: 2

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