[MLton] funny error message

[Andreas Rossberg]

Henry Cejtin wrote:
> I am trying to convert some old SML (1990) code to SML 1997, and MLton is
> printing the following error out:
> 	Variable type in structure disagrees with signature.
> 	variable: flatten
> 	structure: ['a] stream stream -> ['a] stream
> 	signature: ['b] stream stream -> ['b] stream
> As near as I can tell, 'a and 'b are both free.  Am I confused or is this
> error message bad?

This is most likely caused by the value restriction, which wasn't yet 
present in today's form in SML 90. That is, the definition of `flatten' 
in the structure is no longer polymorphic according to the new rules. 
Usually, eta-expansion is sufficient to work around this (it is likely 
that the old code implements `flatten' by partial application of 
something like fold - just turn it into a fun declaration).

-- 
Andreas Rossberg, rossberg@ps.uni-sb.de

Let's get rid of those possible thingies!  -- TB