Hi Conal, The difference is that the LHS of the first rule is mentions the `C k` constraint (probably unintentionally):
*RuleFail> :t morph comp
morph comp :: C k => k1 (k b c) (k a b -> k a c)
but the LHS of the second rule side does not:
*RuleFail> :t morph addC
morph addC :: Num b => k (b, b) b
A work-around is to add the constraint to `morph`:
morph :: D k b => (a -> b) -> k a b
morph = error "morph: undefined"
but I fear that this work-around is not acceptable to you.
Joachim
Am Montag, den 02.10.2017, 14:25 -0700 schrieb Conal Elliott:
> -- 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?
>
> _______________________________________________
> Glasgow-haskell-users mailing list
> Glasgow-haskell-users@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
--
Joachim Breitner
m...@joachim-breitner.de
http://www.joachim-breitner.de/
signature.asc
Description: This is a digitally signed message part
_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
