Goswin von Brederlow wrote:
> "David Allsopp" <dra-news@metastack.com> writes:
> 
> > external foo_of_bar : bar -> foo = "%identity"
> >
> > in *both* the .ml and .mli file for the module in question. I'm
> virtually certain that ocamlopt eliminates calls to the %identity
> primitive.
> 
> Where is that documented?

The use of external and its benefit in exported signatures is documented in
Chapter 18 of the manual.

I believe I became aware of the %identity special handling as a side-effect
of the discussion in
http://caml.inria.fr/pub/ml-archives/caml-list/2007/04/200afc54d9796dbb7a8d7
5bef70f2496.en.html.

If you look in asmcomp/cmmgen.ml you'll see what I think is the elimination
of Pidentity calls in the code generator.

> I would have written
> 
> let foo_of_bar (x : bar) = (x :> foo)

We're solving two different problems, I think (as you're assuming that bar
can be constrained to type foo) - I'm assuming that all type constraining is
done in the signature of the interface so, without %identity I would simply
have put [let foo_of_bar x = x] of type 'a -> 'a in my module... and then
constrained it as [val foo_of_bar : bar -> foo] in the signature.

It is possible that ocamlopt can recognise (obvious) identity functions as
well, but that's just speculation on my part as I haven't looked.


David