[CakeML-dev] Array library

[Ramana Kumar]

I prefer making fromList and tabulate primitives. Maybe some day they could
then be more efficient primitives than the obvious implementation in
source_to_mod. The other two options seem to make empty arrays too special.

On 13 April 2017 at 01:44, Scott Owens <S.A.Owens at kent.ac.uk> wrote:

> At the moment we define Array.fromList and Array.tabulate as follows,
> which is operationally correct, but the type inferencer gives them types
> that are too specific: int list -> int array and int -> (int -> int) -> int
> array.
>
>   fun fromList l =
>     let val arr = array (List.length l) 0
>       fun f l i =
>        case l of
>           [] => arr
>         | (h::t) => (update arr i h; f t (i + 1))
>     in f l 0 end
>
>  fun tabulate n f =
>     let val arr = array n 0
>       fun u x =
>         if x = n then arr
>         else (update arr x (f x); u (x + 1))
>     in
>       u 0
>     end
>
> This is because of the initialisation of the array to 0, before
> overwriting all of the 0s with the real values. Now I can update the code
> to initialise the array with one of the values that will go into the array
> (the head of the list, and f 0, for example). However, this is not possible
> for fromList [] and tabulate 0, which both produce empty arrays, and the
> type of the contents are not constrained. I can see several ways to fix
> this, but none are great.
>
> - Make fromList and tabulate be primitives in the source semantics (which
> could be implemented in source_to_mod rather than a later phase).
> - Add a new array_empty primitive with type unit -> ‘a array
> - Make an attempt to create an empty array raise an exception
>
> Any preferences?
>
> Scott
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