[CakeML-dev] Array library

[Scott Owens]

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.

Scott

> 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))
>     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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