This is all expected behavior. GHC's lazy overlap checking for class instances simply cannot apply to type families -- it would be unsound. I'm afraid I don't see what can be improved here.
Richard
On Jun 6, 2015, at 2:04 AM, AntC
From: AntC Date: 2015-06-04 22:39:25 GMT
Take the standard example for partial overlaps. Suppose I have a class: ...
I'm also getting (in more complex examples) GHC complaining it can't infer the types for the result of f. So now I'm having to put type equality constraints on the class instances, to assure it that F comes up with the right type.
In a reduced example, I'm still getting poor type checking. This is GHC 7.8.3. This seems so dumb, I'm suspecting a defect, It's similar to but much more glaring than: https://ghc.haskell.org/trac/ghc/ticket/10227 https://ghc.haskell.org/trac/ghc/ticket/9918
{-# LANGUAGE TypeFamilies, FlexibleInstances #-} module ClosedTypeFamily where
data Foo b c = Foo b c deriving (Eq, Read, Show)
type family F a where F (Foo Int c) = Int -- Foo Int is first instance F (Foo b Char) = Char
class C a where f :: a -> F a
instance C (Foo Int c) where -- compiles OK f (Foo x _) = x
instance (F (Foo b Char) ~ Char) => C (Foo b Char) where f (Foo _ y) = y
needs the eq constraint. Without it, GHC complains: Couldn't match expected type ‘F (Foo b Char)’ with actual type ‘Char’ Relevant bindings include f :: Foo b Char -> F (Foo b Char) In the expression: y In an equation for ‘f’: f (Foo _ y) = y
Note that if I change the sequence of the family instances for F, then GHC instead complains about the class instance for (Foo Int c).
OK these are overlapping class instances. But GHC's usual behaviour (without closed type families) is to postpone complaining until and unless a usage (Foo Int Char) actually turns up.
BTW if I put a first family instance F (Foo Int Char) = Int to explicitly catch the overlap, then GHC complains about **both** class instances.
Reminder [to Richard] I need not only types but also terms.
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