#1426: warning about `import Control.Monad.Fix` being unused when mdo is used
---------------------------+------------------------------------------------
Reporter: Isaac Dupree | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.6.1
Severity: normal | Keywords:
Difficulty: Unknown | Os: Unknown
Testcase: | Architecture: Unknown
---------------------------+------------------------------------------------
and the use is exported. This should not be warned about since
Control.Monad.Fix is supposed to be imported in modules that use mdo, to
be portable or something. "You should import Control.Monad.Fix. (Note:
Strictly speaking, this import is required only when you need to refer to
the name MonadFix in your program, but the import is always safe, and the
programmers are encouraged to always import this module when using the
mdo-notation.)"
{{{
{-# OPTIONS_GHC -fglasgow-exts -fwarn-unused-imports #-}
module Test where
import Control.Monad.Fix
isEven :: Int -> Maybe Int
isEven n = if even n then Just n else Nothing
puzzle :: [Int]
puzzle = mdo (x, z) <- [(y, 1), (y^2, 2), (y^3, 3)]
Just y <- map isEven [z+1 .. 2*z]
return (x + y)
}}}
produces
{{{
Test.hs:5:0:
Warning: Module `Control.Monad.Fix' is imported, but nothing from it
is used,
except perhaps instances visible in `Control.Monad.Fix'
To suppress this warning, use: import Control.Monad.Fix()
}}}
Just doing `import Control.Monad.Fix()` wouldn't fulfill the purpose of
importing that module!
I wonder if there are any other special syntaxes (arrows maybe?) with this
effect.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1426>
GHC <http://www.haskell.org/ghc/>
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