id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
1897	Ambiguous types and rejected  type signatures	guest	chak	"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  (haskell@zeuxis.de)


"	bug	closed	low	7.2.1	Compiler (Type checker)	6.9	fixed		andres@… dsf@… hackage.haskell.org@…	Linux	x86	None/Unknown	Unknown	typecheck/should_compile/T1897a, indexed_types/should_compile/T1897b		5296