#3691: Error message doesn't mention necessary extension in warning
---------------------------------+------------------------------------------
Reporter: arsenm | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.10.4 | Keywords:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: Incorrect warning at
compile-time
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In this example(Taken from
[http://www.haskell.org/haskellwiki/GHC/AdvancedOverlap]) with the
exception of removing -fglasgow-exts), GHC gives the wrong error message
and fails to mention the missing necessary extension (which turns out to
be ScopedTypeVariables).
If you take the example, and remove -fglasgow-exts and try to only use the
minimum required language extensions,
{{{
{-# LANGUAGE EmptyDataDecls,
MultiParamTypeClasses,
FunctionalDependencies,
OverlappingInstances,
FlexibleInstances,
UndecidableInstances #-}
}}}
GHC fails with the error
{{{
wiki.hs:30:12:
Could not deduce (Print' flag a)
from the context (Print a, ShowPred a flag1, Print' flag1 a)
arising from a use of `print'' at wiki.hs:30:12-35
Possible fix:
add (Print' flag a) to the context of the instance declaration
In the expression: print' (undefined :: flag)
In the definition of `print': print = print' (undefined :: flag)
In the instance declaration for `Print a'
Failed, modules loaded: none.
}}}
However, it compiles fine if ScopedTypeVariables is added to the list of
extensions.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3691>
GHC <http://www.haskell.org/ghc/>
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