CODEWITHSUNDEEP
ats
user7039610
user7039610

Reputation:

How to write a loop over a given list in ATS?

Suppose I have a list xs. How do I write the following style of loop in ATS:

foreach x in xs do process(x)

Upvotes: 3

Views: 159

Answers (3)

Hongwei Xi
Hongwei Xi

Reputation: 935

This is a POOR solution one needs to avoid!

A beginner of functional programming often does list traversal using the list_get_at function (which overloads the symbol []). For instance, it is fairly common to see code that does more or less what the following line does:

(length(xs)).foreach()(lam i => process(xs[i])) // O(n^2)-time

This is EXTREMELY inefficient. Calling list_get_at inside a loop is almost always a bad idea!

Upvotes: 0

Hongwei Xi
Hongwei Xi

Reputation: 935

A combinator-based solution (for a list0-value):

(xs).foreach()(lam x => process(x))

In ATS, foreach is overloaded with many functions that do some form of sequence traversal.

There is also iforeach if the position of each element is needed:

(xs).iforeach()(lam (i, x) => process(i, x))

Upvotes: 0

Artyom Shalkhakov
Artyom Shalkhakov

Reputation: 1101

You can use the old DIY-style (also: the classical ATS style), that is to say, using a tail-recursive function. Here's an example:

extern
fun
process (x: int): void
fun
loop {n:int} (xs: list(int, n)): void =
  case+ xs of
  | list_nil () => ()
  | list_cons (x, xs1) => let
    val () = process (x)
  in
    loop (xs1)
  end
// end of [loop]

You can run the full code online

I think that this approach is preferable if none of the combinators or template functions provided in the libraries such as ATSLIB is suitable for your case.

Upvotes: 1

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