[GHC] #13957: Allow deriving multiparameter type classes with representationally equal arguments
GHC
ghc-devs at haskell.org
Tue Jul 18 18:45:56 UTC 2017
#13957: Allow deriving multiparameter type classes with representationally equal
arguments
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Reporter: Iceland_jack | Owner: (none)
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 8.0.1
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
| Unknown/Multiple
Type of failure: None/Unknown | Test Case:
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s):
Wiki Page: |
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Comment (by Iceland_jack):
We cannot both have `SIEVE I (->)` and `SIEVE Identity (->)` if we assume
a functional dependency, similar to #13404 where an analogous example
would compile
{{{#!hs
class SIEVE p where
type SIEVE_ty p :: Type -> Type
sIEVE :: p a b -> (a -> SIEVE_ty p b)
instance SIEVE (->) where
type SIEVE_ty (->) = Identity
sIEVE :: (a -> b) -> (a -> Identity b)
sIEVE f = f >>> Identity
newtype ARR a b = ARR (a -> b)
deriving newtype
SIEVE
}}}
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13957#comment:4>
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