#4816: DeriveFunctor should not require a functor instance for functorial data
types used invariantly
---------------------------------+------------------------------------------
Reporter: batterseapower | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.12.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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This program:
{{{
{-# LANGUAGE DeriveFunctor #-}
data Silly a = Sillly a
data Baz o = Baz {
foo :: o,
bar :: Silly ()
} deriving (Functor)
}}}
Produces this error:
{{{
/Users/mbolingbroke/Junk/Repro2.hs:7:14:
No instance for (Functor Silly)
arising from the 'deriving' clause of a data type declaration
at /Users/mbolingbroke/Junk/Repro2.hs:7:14-20
Possible fix:
add an instance declaration for (Functor Silly)
or use a standalone 'deriving instance' declaration instead,
so you can specify the instance context yourself
When deriving the instance for (Functor Baz)
}}}
But there is absolutely no need for such an instance, because I'm not
using "Silly a", just "Silly ()".
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4816>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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