On Tue, May 02, 2006 at 12:26:48AM +0100, Ross Paterson wrote:
> So the MR raises the reverse
> question: can you be sure that every tautologous constraint is reducible?
I think the answer is no. Consider:
class C a where
type Result a
method :: a -> Result a
instance C (Maybe a) where {
type Result (Maybe a) = Bool
f = \ b x -> if b then Just (method x) else x
The AT algorithm will stop after inferring the following type for f:
(a = Maybe (Result a), C a) => Bool -> a -> a
Since the constraint set is non-empty, this definition will be rejected
by the monomorphism restriction.
However, this constraint set has exactly the same set of solutions as
a = Maybe Bool, which is trivially solvable. (The corresponding FD
version will reduce to this, thanks to instance improvement.)
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