Question

I am reading DrRacket document http://docs.racket-lang.org/guide/binding.html

There is a function

  (define f
    (lambda (append)
      (define cons (append "ugly" "confusing"))
      (let ([append 'this-was])
        (list append cons))))
  > (f list)
  '(this-was ("ugly" "confusing"))

I see that we define function f, inside we define lambda that takes (append), why ?
Procedure (body) for lambda is another function called cons, that appends two strings.

I don’t understand this function at all.
Thanks !

Answer 1

Scheme takes some getting used to 🙂

1. f is assigned the function returned by the lambda.
2. lambda defines the function that takes a parameter (called append).
3. (define cons (append "ugly" "confusing")) is not a function per se, but calls append with the two strings as parameter and assigns the result to cons.
4. inside the let block, append is re-assigned a different value, the symbol this-was.
5. the let block creates a list of append (which now contains 'this-was) and cons (which contains '("ugly" "confusing") from 3 above
6. since 5 is the last statement that value is returned by the whole function which is called f
7. f is called with the parameter list (the list function). which gets passed as the parameter append. And this is why 3 above creates a list '("ugly" "confusing") which gets assigned to cons.

Hope that cleared up things a bit.
Cheers!