#3691: Error message doesn't mention necessary extension in warning
------------------------------------------------+---------------------------
Reporter: arsenm | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler | Version: 6.10.4
Resolution: invalid | Keywords:
Difficulty: | Os:
Unknown/Multiple
Testcase: | Architecture:
Unknown/Multiple
Failure: Incorrect warning at compile-time |
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Changes (by simonpj):
* status: new => closed
* difficulty: =>
* resolution: => invalid
Comment:
The error message looks right to me. GHC can't deduce `(Print' flag a)`
from `(Print' flag1 a)`. You may ask why it doesn't choose `flag1` to be
equal to `flag`. It doesn't because it's too hard for the type inference
engine to do. The declaration
{{{
instance (ShowPred a flag, Print' flag a) => Print a where
print = print' (undefined::flag)
}}}
tries to do so, by using "`flag`" in both places, but under Haskell 98
rules the type signature on `undefined` means `undefined :: forall flag.
flag`, and that is not what you want.
So the language extensions you supplied are not "the minimum required";
you need `ScopedTypeVariables`.
I've modified the HaskellWiki page to use LANGUAGE pragmas.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3691#comment:1>
GHC <http://www.haskell.org/ghc/>
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