Cryptic type error message (should be syntax error)

It seems ghc allows a syntax for types which if beyond Haskell98 even without -fglasgow-exts. Consider the following program:

destroy :: (forall a. (a -> Maybe (b,a)) -> a -> c) -> [b] -> c
destroy g xs = g listpsi xs
  where listpsi :: [a] -> Maybe (a,[a])
        listpsi [] = Nothing
        listpsi (x:xs) = Just (x,xs)

It gives the following rather cryptic error message without -fglasgow-exts:

ConcatMap.hs:7:17:
    Couldn't match expected type `.' (a rigid variable)
           against inferred type `(->)'
      `.' is bound by the type signature for `destroy'
        at ConcatMap.hs:6:20
    Probable cause: `listpsi' is applied to too few arguments
    In the first argument of `g', namely `listpsi'
    In the expression: g listpsi xs

The fact that ghc interpreted `.' as a type variable baffeled me for a moment. I would expect ghc to report a syntax error since the type is not syntactically Haskell98.

Trac metadata

Trac field	Value
Version	6.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown

changed weight to 5

added Tbug Trac import labels

Haskell 98 indeed does not seem to allow operators as types, so we ought not allow this without -fglasgow-exts. Too late for 6.6, though...

Trac metadata

Trac field	Value
Component	Compiler → Compiler (Type checker)

added typechecker label

Fixed thank you. Test is tcfail166

Simon

mentioned in commit b2d6de1c

mentioned in commit 7fbb69b2

closed

Definitely fixed, error is now:

919.hs:1:20:
    Illegal operator `.' in type `forall a . ((a -> Maybe (b, a))
                                              -> a -> c)'
      (Use -XTypeOperators to allow operators in types)

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

changed milestone to %6.8.1

Trac metadata

Trac field	Value
Architecture	Unknown → Unknown/Multiple

Trac metadata

Trac field	Value
Operating system	Unknown → Unknown/Multiple

added Pnormal label