#1897: Ambiguous types and rejected type signatures
----------------------------------------+-----------------------------------
Reporter: guest | Owner: chak
Type: bug | Status: reopened
Priority: normal | Milestone: 6.10 branch
Component: Compiler (Type checker) | Version: 6.9
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Os: Linux
Architecture: x86 |
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Changes (by simonpj):
* summary: Type families: type signature rejected => Ambiguous types and
rejected type signatures
Old description:
> 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])
New description:
This ticket collects a bunch of examples where GHC
* Rejects a function with a type signature but, if the type signature is
removed, accepts the function and infers precisely the type that was
originally specified.
* Accepts type signatures that are utterly ambiguous; that is, the
function could never be called, or only in the presence of bizarre
instance declarations.
* Rejects definitions that clearly have a unique typing.
Original example: 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#comment:9>
GHC <http://www.haskell.org/ghc/>
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