[MLton] building mlton on Snow Leopard

[Matthew Fluet]

On Thu, 17 Sep 2009, John Reppy wrote:
> I'm trying to compile mlton (revision 7228) using the command
>
> 	make TARGET_ARCH=x86_64 smlnj-mlton
>
> on Snow Leopard, but I'm hitting type errors in ssa/combine-conversions.fun

Nothing to do with Snow Leopard.  Everything to do with SML/NJ using an 
overly restrictive context for type inference.

> ssa/combine-conversions.fun:57.5-58.60 Warning: type vars not generalized 
> because of value restriction are instantiated to dummy types (X1,X2,...)

The code in question is:

val { get, set, ... } =
    Property.getSetOnce (Var.plist, Property.initConst NONE)

Yes, the type of NONE (and get and set) are undetermined by this 
declaration.  However, subsequent use of get and set within the module 
determine the types.

For a smaller example:

structure S =
struct
    val z = (fn x => x) []
    val y = z :: [true] :: nil
end

Accepted by MLton, Poly/ML, and HaMLet; only rejected by SML/NJ.

Another example:

structure S : sig val z : bool list end =
struct
    val z = (fn x => x) []
end

Accepted by MLton, Poly/ML, and HaMLet; only rejected by SML/NJ.


Admittedly, we have attempted to keep the MLton sources compatible with 
SML/NJ, so we will probably put in an explicit type constraint (you'll see 
such a constraint in most uses of Property.* functions), but it's really 
SML/NJ's bug.