Re: [Caml-list] Purity in ocaml

[Goswin von Brederlow <goswin-v-b@web.de>]

On Thu, Jan 23, 2014 at 10:35:14AM +0100, Arnaud Spiwack wrote:
> > On the other hand if types are annotated with the exceptions they
> > throw then they become just another return value and become pure.
> > Same input, same exception every time.
> >
> 
> As far as Yotam Barnoy's goal is concerned, this isn't true. He wants to be
> able to use purity annotations for optimisation: a most typical
> optimisation which is valid only for pure functions is to rewrite *map f
> (map g l)* into *map (f???g) l*. This optimisation, as it happens, is not
> valid with exception throwing functions as it may very well throw a
> different exception.

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. 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

MfG
	Goswin