#2118: deriving for GRose
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Reporter: alexey | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.8.2
Severity: normal | Resolution:
Keywords: type classes, undecidable instances | Testcase:
Architecture: x86 | Os: MacOS X
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Comment (by fons):
Quoting the ghc 6.8.2 manual [1]
''
"The rule is this: each constraint in the inferred instance context must
consist only of type variables, with no repetitions. This rule is applied
regardless of flags. If you want a more exotic context, you can write it
yourself, using the standalone deriving mechanism."''
And indeed, using the standalone deriving mechanism (flag
-XStandaloneDeriving) we can create a Show instance of GRose:
{{{
{-# LANGUAGE StandaloneDeriving #-}
data GRose ff a = GRose a (ff (GRose ff a))
deriving instance (Show a, Show (ff (GRose ff a))) => Show (GRose ff a)
}}}
[1] http://www.haskell.org/ghc/docs/6.8.2/html/users_guide/deriving.html
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2118#comment:1>
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