Re: [Caml-list] Purity in ocaml

[Arnaud Spiwack <aspiwack@lix.polytechnique.fr>]

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> Do you mean something like this?
>
> let g = function 0 -> raise G | n -> n
> let f = function 1 -> raise F | n -> n
>
> map f (map g [1;0]) ===> raise G
> map (f %> g) [1;0]  ===> raise F
>
> It is true, the exception prevents any optimization that would reorder
> two functions that both throw an excepition.


Precisely


> But many other
> optimizations are still possible, e.g. common subexpression
> elimination:
>
> let n =
>   let x = f (g 1) in
>   let y = g 1 in
>   x + y
>
> ==>
>
> let n =
>   let t = g 1 in
>   let x = f t in
>   let y = t in
>   x + y
>

To some degree:

let n =
  let x = f (g 1) (h ()) in
  let y = g 1 in
  x+y

is a different program than

let n =
  let t = g 1 in
  let x = f t (h ()) in
  let y = t in
  x+y

As it reorders (g 1) and (h ()). I think the rule is something like: when
the program is in monadic form, and you have a call to e, you can factor in
every subsequent calls to e. Which I guess is good enough. But I would tend
to believe deforestation would be tremendously more useful than common
expression elimination.


>
> MfG
>         Goswin
>
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