[CakeML-dev] Array library
S.A.Owens at kent.ac.uk
Thu Apr 13 16:55:24 UTC 2017
That’s also the hardest option to implement, since it involves changing the structure of the source_to_mod proof. I currently favour adding an empty array allocation primitive because I think it is the easiest option.
> On 2017/04/13, at 07:16, Ramana Kumar <Ramana.Kumar at cl.cam.ac.uk> wrote:
> 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))
> u 0
> 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?
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