Re: [Sml-implementers] type-checking bug in several implementations

[Stephen W. <sw...@sw...>]

> Alice also rejects it, with a halfway decent error message:
> 
> 1.10-12.4: module expression does not match signature:
>    val f : '1 option -> '1 option
> does not match
>    val f : 'a option -> 'a option
> because
>    '1
> is incompatible with
>    '_a
> 
> Actually, I fixed this only a couple of months ago, using some variable
> level hack. Somehow I forgot to double-check Hamlet on this occasion. I
> doubt that I can use the same hack there.

I ended up patching MLton using the same code that checks if explicit
type variables in val decs can be generalized.  The error message
looks like

Error: z.sml 6.4: variable type in structure disagrees with signature
   variable: f
   unable to generalize: 'a
   signature: 'a option -> 'a option