I'm running into type checking problem with GHC rewrite rules in GHC 8.0.2, illustrated in the code below. The first rule type-checks, but the second triggers the type error mentioned in the comment following the rule definition. I'd expect both rules to type-check, adding the needed constraints to the applicability condition for the rule.
Is GHC's behavior intended (and perhaps necessary), or a bug?
Thanks, -- Conal
Code (also attached):
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# OPTIONS_GHC -Wall #-}
-- Demonstrate a type checking failure with rewrite rules
module RuleFail where
class C k where comp' :: k b c -> k a b -> k a c
instance C (->) where comp' = (.)
-- Late-inlining version to enable rewriting.
comp :: C k => k b c -> k a b -> k a c
comp = comp'
{-# INLINE [0] comp #-}
morph :: (a -> b) -> k a b
morph = error "morph: undefined"
{-# RULES "morph/(.)" morph comp = comp #-} -- Fine
class D k a where addC' :: k (a,a) a
instance Num a => D (->) a where addC' = uncurry (+)
-- Late-inlining version to enable rewriting.
addC :: D k a => k (a,a) a
addC = addC'
{-# INLINE [0] addC #-}
{-# RULES "morph/addC" morph addC = addC #-} -- Fail
-- • Could not deduce (D k b) arising from a use of ‘addC’
-- from the context: D (->) b
-- Why does GHC infer the (C k) constraint for the first rule but not (D k
b)
-- for the second rule?
RuleFail.hs
Description: Binary data
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