#1897: Type families: type signature rejected
----------------------+-----------------------------------------------------
Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.9
Severity: normal | Keywords:
Testcase: | Architecture: x86
Os: Linux |
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The following programm does not compile:
{{{
{-# LANGUAGE TypeFamilies #-}
import Control.Monad
import Data.Maybe
class Bug s where
type Depend s
next :: s -> Depend s -> Maybe s
start :: s
isValid :: (Bug s) => [Depend s] -> Bool
isValid ds = isJust $ foldM next start ds
}}}
Error:
{{{
GHCi, version 6.9.20071105: http://www.haskell.org/ghc/ :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Main ( GHC-Bug.hs, interpreted )
Bug.hs:13:39:
Couldn't match expected type `Depend a'
against inferred type `Depend s'
Expected type: [Depend a]
Inferred type: [Depend s]
In the third argument of `foldM', namely `ds'
In the second argument of `($)', namely `foldM next start ds'
Failed, modules loaded: none.
}}}
When one elides the type signature the program compiles yielding the
offending signature:
{{{
Prelude> :r
[1 of 1] Compiling Main ( GHC-Bug.hs, interpreted )
Ok, modules loaded: Main.
*Main> :t isValid
isValid :: (Bug a) => [Depend a] -> Bool
}}}
Compiler version: 6.9.20071105
Best regards,
Harald Holtmann ([EMAIL PROTECTED])
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1897>
GHC <http://www.haskell.org/ghc/>
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