#4815: Instance constraints should be used when deriving on associated data
types
---------------------------------+------------------------------------------
Reporter: batterseapower | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.12.3
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Comment(by simonpj):
It's not clear that you always want this. Suppose your data decl was
{{{
instance Foo a => Foo (Maybe a) where
data Associated (Maybe a) = AssociatedMaybe a
deriving (Eq)
}}}
Here, `Eq a` would be enough. Wouldn't you expect this
{{{
instance Eq a => Eq (Associated (Maybe a))
}}}
rather than
{{{
instance Foo a => Eq (Associated a)
}}}
? Maybe standalone deriving is the Right Way, not just a workaround.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4815#comment:1>
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