#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 |
---------------------------+------------------------------------------------
Changes (by chak):
* summary: Associated types missing useful functionality => Improve
error message if an associated family
declaration has excess parameters
Comment:
No, it's not a bug. The second example ought to be written:
{{{
class C2 f where
type T2 f :: * -> *
op2 :: T2 f e -> Either e a
}}}
The parameters of an associated family declaration must be a subset of the
class parameters as documented at
[http://haskell.org/haskellwiki/GHC/Type_families#Associated_family_declarations_2].
However, remember that
{{{
type family S1 a :: * -> *
}}}
and
{{{
type family S2 a b :: *
}}}
are '''not''' the same thing. `S1` has one type index (and in each
occurrence must be applied to at least one argument), whereas `S2` has two
type indices (and in each occurrence must be applied to at least two type
arguments). (BTW, I'm not saying that this is the only design that's
possible, but it is what we decided to be most useful at the time.)
However, the error message can certainly be improved!
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3714#comment:1>
GHC <http://www.haskell.org/ghc/>
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