Chris Hecker wrote:
> 
> >        Because types in a given implementation may or may not be present
> >in the interface.
> 
> Right, that's fine, but I only want to have them once, whereever they are.  So, if it's a private type, it's in the ml file, and if it's public it's in the mli file (assuming I want one, if I don't then everything's public).  How does having them duplicated buy me anything but trouble?
> 
> Chris

In C++, a function is either extern or static. 
There are only two options. [And non-local classes are _always_ extern]
This is not so in Ocaml: you may wish to provide access to a component
such as a function with a type more constrained than the actual
implementation.
For example:

	(* twoint.mli *)
	val twoint: int -> int * int

	(* twoint.ml *)
	let twoint x = (x,x)

Here, the implementation is actually polymorphic, it will work
for more than ints, but clients of the module 'twoint' cannot
use this function to make a pair of floats.

Here is another example:

	(* abstr.mli *)
	type t
	val get: t -> int
	val make: int -> t

	(* abstr.ml *)
	type t = int * int
	let get x = fst x
	let make x = (x,x+1)	


Here, the type t is implemented as a pair of ints.
But the implementation is hidden from the client of the module.
Note that the type 't' itself is not hidden.

-- 
John (Max) Skaller, mailto:skaller@maxtal.com.au
10/1 Toxteth Rd Glebe NSW 2037 Australia voice: 61-2-9660-0850
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