CODEWITHSUNDEEP
lambdaschemeracketlet
stack93m
stack93m

Reputation: 115

Rewrite an expression using let

I have been teaching scheme language and in my textbook I hit on the below expression. My question is why it is evaluating on number '10'? Can someone please explain it to me? Meaning what local environment are created and what bindings are created to symbols. Also I know that every lambda expression can be rewritten using let. Any idea how to do that? I would appreciate any help.

(let ((x 10)) 
  (define y (lambda (x) x)) 
  (define x 5) 
  (+ x (y x)))

Upvotes: 2

Views: 227

Answers (1)

Sylwester
Sylwester

Reputation: 48745

Your code in a more readable form:

(let ((x 10))
  (define y (lambda (x) x))
  (define x 5)
  (+ x (y x)))

You can apply the substitution method:

(+ x (y x)) ; ==>
(+ 5 (y 5)) ; ==>
(+ 5 ((lambda (x) x) 5)) ; ==>
(+ 5 5) ; ==>
10

You got it the wrong way. Every let can be rewritten as a lambda which is immediately called:

(let ((a aexpr) (b bexpr))
  body ...)

; ==

((lambda (a b) body ...) aexpr bexpr)

Upvotes: 5

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