Re: Subtyping structurally-equivalent records, or something like it?

[Sylvain Le Gall <sylvain@le-gall.net>]

On 04-05-2010, AUGER Cédric <Cedric.Auger@lri.fr> wrote:
> I am not expert in Ocaml, is the following the same in terms
> of performances as the phantom types?
>
> type kinematic = ...
>
> type force = Force of kinematic
> type momentum = Moment of kinematic
> ...
>
> That is does the constructor introduce an overhead or not?
> As there is only one constructor, no overhead should be done in an
> optimized compiler.
>

The variants are represented using a block. If you introduce a single
variant, it will create a block that points to kinematic. E.g. "Force of
kinematic" will create a pointer to the kinematic structure. 

Your construction of force and momentum will add a level of indirection
for every use of the kinematic structure.

Phantom type doesn't add a level of indirection and left no trace in the
generated assembler. 

This is not about optimized compiler in this case but about data
representation. Even if you use an optimized compiler (which is not
really the case with ocamlopt), you won't change datastructure
representation to optimize.

Regards,
Sylvain Le Gall