[Haskell-cafe] Equality constraints in type families

[Manuel M T Chakravarty]

Simon Peyton-Jones:
> | > * GHC says that these constraints must be obeyed only
> | >        *after* the programmer-written type has been normalised
> | >        by expanding saturated type synonyms
> | >
> ...
> | > I regard this as a kind of pre-pass, before serious type checking
> | > takes place, so I don't think it should interact with type  
> checking
> | > at all.
> | >
> | > I don't think this normalisation should include type families,
> | > although H98 type synonyms are a kind of degenerate case of type
> | > families.
> | >
> | > Would that design resolve this particular issue?
> |
> | Not quite, but it refines my proposal of requiring that type  
> synonyms
> | in the rhs of type instances need to be saturated.  Let me  
> elaborate.
>
> Why not quite?

Maybe I was thinking too much in terms of GHC's implementation, but  
due to the lazy expansion type synonyms, the expansion is interleaved  
with all the rest of type checking.  But I think I now know what you  
meant: the outcome should be *as if* type synonym expansion was done  
as a pre-pass.

> | So, the crucial point is that, as you wrote,
> |
> | > I don't think this normalisation should include type families,
> | > although H98 type synonyms are a kind of degenerate case of type
> | > families.
>
> Exactly!   Just to state it more clearly again:
>
>        Any programmer-written type (i.e one forming part
>        of the source text of the program) must obey the
>        following rules:
>        - well-kinded
>        - type synonyms saturated
>        - arguments of type applications are monotypes
>                (but -> is special)
>
>        However these rules are checked ONLY AFTER EXPANDING
>        SATURATE TYPE SYNONYMS (but doing no reduction on
>        type families)

I agree.

> The above checks are performed by checkValidType in TcMType.  In  
> particular, the check for saturated synonyms is in check_type (line  
> 1134 or thereabouts).  I'm not sure why these checks are not firing  
> for the RHS of a type family declaration.  Maybe we aren't calling  
> checkValidType on it.

I'll check that.  Might be an oversight.

> So I think we are agreed.  I think the above statement of validity  
> should probably appear in the user manual.

Yes, I'll take care of that.

Manuel