#3714: Improve error message if an associated family declaration has excess
parameters
---------------------------+------------------------------------------------
Reporter: simonpj | Owner: chak
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.10.4
Resolution: | Keywords:
Difficulty: | Os: Unknown/Multiple
Testcase: | Architecture: Unknown/Multiple
Failure: None/Unknown |
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Comment (by MartijnVanSteenbergen):
Moving the type index {{{e}}} to the right of the {{{::}}}—turning it into
a type parameter, like Manuel suggests—complicates writing certain
instances. Before I could write:
{{{
type instance T2 () e = e -> Bool
}}}
But now I have to write:
{{{
instance C2 () where
type T2 () = T2Unit
newtype T2Unit e = T2Unit (e -> Bool)
}}}
introducing a newtype whenever the type is not directly eta-reducible. In
my case this defeats the purpose of writing these instances, as the goal
is to derive a ''simpler'' type from a generic representation of a
datatype (instances for generic building blocks), not a more complicated
one (with newtypes that weren't necessary before).
Just to be clear, there isn't really a problem, because I can just leave
the type family outside the class. So the actual question here is: why are
[type family inside type class] and [multiple family indices for writing
type lambdas] sometimes mutually exclusive?
Thanks,
Martijn.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3714#comment:3>
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