#2885: Late and confusing error on uncallable class method
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Reporter: blamario | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.10.1 | Severity: minor
Keywords: | Testcase:
Os: Linux | Architecture: x86_64 (amd64)
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This has been discussed on Haskell Café (http://www.haskell.org/pipermail
/haskell-cafe/2008-December/051856.html), I'm reporting it here in case
something can be done to help other confused souls in future.
In short, the error message reported by GHC (6.10.1):
{{{
Could not deduce (Container x y) from the context (Container x y1)
arising from a use of `wrapper' at Test.hs:11:22-30
Possible fix:
add (Container x y) to the context of
the type signature for `liftWrap'
In the expression: wrapper x
In the expression:
(if wrapper x then rewrap . f . unwrap else id) x
In the definition of `liftWrap':
liftWrap f x = (if wrapper x then rewrap . f . unwrap else id) x
}}}
does not begin to indicate that the actual problem with the following
program is in the type class declaration:
{{{
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
import Data.Maybe
class Container x y where
wrapper :: x -> Bool
unwrap :: x -> y
rewrap :: y -> x
liftWrap :: Container x y => (y -> y) -> (x -> x)
liftWrap f x = (if wrapper x then rewrap . f . unwrap else id) x
instance Container (Maybe x) x where
wrapper = isJust
unwrap = fromJust
rewrap = Just
main = print (liftWrap (succ :: Int -> Int) (Just 1 :: Maybe Int))
}}}
If the 'wrapper' method can never be called in any possible context, there
should be an error report at the point where the method is declared. I
don't think that's a controversial statement. Even if the compiler allows
the declaration under the assumption that the useless method is never
called, it should at least emit a strong warning.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2885>
GHC <http://www.haskell.org/ghc/>
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