Relax superclass restrictions

Doaitse points out that we currently reject

class C a b | a -> b where

class C a b => D a  where

on the grounds that 'b' is not in scope in the second class decl. (Only type variables in the "head", namely (D a), can be mentioned in the superclass context.) My response to him was as follows.

The easiest way forward is to re-express your program using type functions. Then class C will have just a single type parameter (a), with the 'b' part being expressed by a type function. That would resolve the problem rather nicely.

Medium term, I think the Right Thing is to allow a class declaration

class Q => C a b

(where Q is a context) if and only iff the type

forall ab. Q => C a b

is unambiguous. What does "unambiguous" mean? As it happens, we are working on nailing that down right now. For example, here is a stupid but unambiguous declaration:

type family F a
class (b ~ F a, Eq b) => C a

I do not know of any non-stupid examples that would be rejected by the current rule, but there might be some. I'll open a ticket because I'd like to get to this when we have the other pieces working.

Trac metadata

Trac field	Value
Version	6.10.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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