[MLton] type error messages

[Matthew Fluet]

John came up with a few signature/structure pairs where the error message
reported could be improved.

signature t = sig
   type y = int
end

structure t : t = struct
   type y = char
end

Error: z.sml 5.15.
  Type definition in structure disagrees with signature.
    type: y
    structure: [y]
    signature: [int]

It would probably be more useful to print out the fact that within the
structure y is bound to char.  We figured out that MLton consistently
prints a type by using the type name closest in scope with the same type.
However, in the above, it is less than illuminating.  When printing out an
error about a particular type name, would it be possible to choose the
scope not including the type name?


signature s = sig
  datatype y = Y
  type x = y
end where type x = int

structure s : s = struct
  datatype y = Y
  type x = int
end

Error: y.sml 6.15.
  Type y admits equality in signature but not in structure.
    not equality:
Error: y.sml 6.15.
  Variable type in structure disagrees with signature.
    variable: Y
    structure: [y]
    signature: [x]

The first error is kind of misleading.  It's really seems as though we
should get a type-error like the first example:

Error: y.sml 6.15.
  Type definition in structure disagrees with signature.
    type: y
    structure: [y]
    signature: [int]

Here, the y in the structure is really the datatype y, so can't be
improved in any meaningful way.

The second error is a little hard to follow, but I don't see any way to
improve it without breaking the "most recent name" convention, which seems
to be useful in other scenarios.