Native review for jonathanhefner/elm-range about package.elm-lang.org HOT 2 CLOSED

I want to keep native code as a last resort for expressiveness, and I think this can be expressed without massive issues in pure Elm.

For example, List.foldl is implemented as a loop under the hood, so it seems that defining your stuff in terms of that would be no problem. To do foldr maybe you want to use List.reverse but we are talking about non-crazy costs here. Does that route seem plausible? If not, why not?

from package.elm-lang.org.

Although Range.foldl could be expressed in terms of List.foldl, it would require allocating an intermediary list. If you want to work with a large range, for example range 0 50000 |> foldl (+) 0, that may be impractical.

But further, I had envisioned using this as a simple and safe looping mechanism for counted loops. As an example, several List API functions which currently require native code (for safety) could be expressed in terms of Range.foldl:

drop : Int -> List a -> List a
drop n xs =
  let drop1 _ xs' =
        case xs' of
          _::rest -> rest
          _ -> []
  in
      0 `to` n |> Range.foldl drop1 xs

repeat : Int -> a -> List a
repeat n x =
  let cons1 _ xs = x :: xs
  in
      0 `to` n |> Range.foldl cons1 []

I agree that we should be eliminating native code wherever possible (for all the reasons stated in elm-lang/core#130). In this case, I felt Range.foldl would actually help move towards that goal.

from package.elm-lang.org.