[MLton] (possible) front-end bug

[Stephen Weeks]

> > I suspect that SML/NJ is correct; a datatype in a signature would be an
> > (implicit) definitional specification, wouldn't it?
> 
> But that can't be quite right either. 

A datatype specification defines a "flexible" tycon just as a type
specification does.

> I do want to be able to say:
> 
> signature T =
> sig
>   structure Y : S where type t = Z.t
>                     and type u = Z.u
> end
> 
> to indicate that any structure matching T will have a substructure Y that
> is essentially equal to Z.

That seems like valid code (which MLton accepts) and to do what you
mean.  The where type does the same thing to the flexible tycon
whether defined by a datatype or type specification -- it replaces the
flexible tycon with the right hand side of the where.

> to the above; the "bug" was a cut-and-paste-and-edit error that ended up
> as the code which MLton accepted but SML/NJ did not.

Yes, the cut-and-paste-and-edit code appears to tickle a MLton bug.  I
am investigating.