Question

I’m trying to implement a function in scheme that splits the given list with the function that is also given as parameter to function. To exemplify:

(splitby '("a" "b" "cc" "ab" "abc" "a" "b")
         (lambda (x y) (= (string-length x) (string-length y))))

should return (("a" "b") ("cc "ab") ("abc") ("a" "b"))

I’m pretty beginner in Scheme so it is really hard to understand how this ‘function like’ parameter works, and while implementing such a function what should I do?

Answer 1

In Scheme, functions are objects like numbers, strings, etc. So in this case, your example is equivalent to this:

(define (equal-length x y)
  (= (string-length x) (string-length y)))

(splitby '("a" "b" "cc" "ab" "abc" "a" "b") equal-length)

The use of the function is to allow the splitting criterion to be customised. In this case, items are in the same group for as long as the given function returns a true value; otherwise a new group is created.

To get started, write a group-equal function, that groups equal elements together:

(define (group-equal lst)
  ...)

where, for example,

(group-equal '(1 2 2 3 3 3 4))

returns

((1) (2 2) (3 3 3) (4))

If you successfully implement that, then it’s identical to your splitby function, except that you use the given function (equal-length, for example) instead of equal? (as group-equal might use).