[MLton] Callback to function pointer?

[Matthew Fluet]

>  _import *: ptrTy -> cfTy;	    ptrTy -> cfTy
>
> So I think it makes sense to
> stick with _import * (which, by the way Matthew must have slipped the
> currying by me in my sleep :-).

I think the currying is very useful here, because the common scenario is 
what appears in examples/ff/iimport.sml:

local
   val double_to_double =
      _import * : DynLink.fptr -> real -> real;
   val sin_fptr = DynLink.dlsym (hndl, "sin")
   val cos_fptr = DynLink.dlsym (hndl, "cos")
   val tan_fptr = DynLink.dlsym (hndl, "tan")
in
   val sin = double_to_double sin_fptr
   val cos = double_to_double cos_fptr
   val tan = double_to_double tan_fptr
end

> Stephen proposed:
> 
>  _symbol "symbol": cbTy;            (unit -> cbTy) * (cbTy -> unit)
>  _symbol *: ptrTy, cbTy;            (ptrTy -> cbTy) * (ptrTy * cbTy -> unit)

My vote is for this proposal.  

>  o is not backward compatible
>  o introduces a new keyword

It is an advanced, unsafe feature, so I think some breakage is o.k.  
Likewise, at most it breaks  _import "symbol": cbTy;  and the undocumented  
_import # "symbol" : ptrTy;

>  o does not use currying

Sigh.  But, I don't think it saves much in this scenario, since the common 
case would be:

local
   val (get_double, set_double) =
      _symbol * : real;
   val epsilon_ptr = DynLink.dlsym (hndl, "epsilon")
in
   val get_epsilon = fn () => get_double epsilon_ptr
   val set_epsilon = fn e => set_double (epsilon_ptr, e)
end

> One thing I don't like about any of the proposals is having to write
> the pointer type, since it is almost always MLton.Pointer.t.  

I disagree.  DynLib would like to use an opaque DynLib.fptr.