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This is related to ticket #821 (closed) (which is indeed fixed in 6.6).

We make extensive use of an (undocumented?) feature of GHC: type synonyms can include implicit parameter constraints such that when the type is used, the constraint 'floats to the top'. This appears to be broken in some cases in 6.6.

Here's an example module:

{-# OPTIONS -fglasgow-exts -fimplicit-params #-}
module TestIP where

type PPDoc = (?env :: Int) => Char

f :: Char -> PPDoc
f c = g $ c

g :: PPDoc -> PPDoc
g d = d

when loaded in ghci 6.6, this yields:

/Users/jefb/Test.hs:7:10:
    Couldn't match expected type `PPDoc' against inferred type `Char'
    Probable cause: `c' is applied to too many arguments
    In the second argument of `($)', namely `c'
    In the expression: g $ c

If g $ c is changed to g c the module compiles.

The example compiles in 6.4.1 (I don't have handy access to other versions right now).

Thanks! Jef

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture	Unknown