Re: [Caml-list] 3.13.0 stricter on interfaces? (building extlib)

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2011/11/26, at 19:33, Gabriel Scherer wrote:

> There is a slighlt misinterpretation in your analysis of the issue:
> the problem comes from the fact that OCaml 3.13 standard library has
> added a ?seed optional parameter to the Hashtbl.create
> method. Extlib's interface still assumes <=3.12 interface with no
> parameter (and in particular didn't change this function interface),
> and tries to check the new Hashtbl.create standard function with the
> old (int -> ('a,'b) t) interface.
> 
> In particular, this problem does *not* come from a change of behavior
> of the typer, only from libraries changing.
> (And the change is on stdlib's side, not extlib's.)

Indeed, the compiler never erases optional arguments to match
a specific signature.

> The OCaml manual is not very explicit about implicit coercions
> regarding optional arguments:
> 
>> Normally the compiler generates a type error if you attempt to pass
>> to a function a parameter whose type is different from the expected
>> one. However, in the specific case where the expected type is
>> a non-labeled function type, and the argument is a function
>> expecting optional parameters, the compiler will attempt to
>> transform the argument to have it match the expected type, by
>> passing None for all optional parameters.
> 
> It seems that in some cases it is able to convert from `?x:foo -> bar`
> to `bar`. Indeed:
> 
>  # (fun (f : int -> int) -> f 3) (fun ?(x=0) y -> x+y);;
>  - : int = 3
> 
> However, this doesn't seem to work when checking interfaces:

Indeed again, as written in the manual, this specific case only
concerns functions passed as function argument. This is not
the case of exports from a module.
More precisely, in 3.12 this applies only to function arguments
and record fields, in trunk this applies also to variant arguments
(both normal and polymorphic) and to explicit type annotations.

> Here are are a few ways this issue could be resolved:
> 
> 1. the stdlib could change to revert to the old interface, adding
>  a `create_seed` function
> 
> 2. extlib could introduce a conditional compilation trick to support
>  either interfaces depending on OCaml version; ugly and painful to
>  deal with in ocamldoc
> 
> 3. instead of repeating `val create : int -> ('a, 'b) t` in its own
>  interface, extlib could include (module type of Hashtbl) (referring
>  to stdlib's Hashtbl module). This way, their interface would keep in
>  synch with stdlib's change, and they would have to change their
>  implementation correspondingly.

There is another solution which works with all versions of OCaml

4. In extHashtbl.ml, add the following line

	let create x = create x

   This will discard optional arguments.
   People who need the full version can get it from the standard library.


Jacques Garrigue