id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
7026,Impredicative implicit parameters,Ashley Yakeley,,"There doesn't seem to be a way to make impredicative implicit parameters work in 7.4.2:

{{{
{-# LANGUAGE ImplicitParams, ImpredicativeTypes #-}
module Bug where

    f1 :: Maybe ((?a :: Bool) => Char)
    f1 = Just 'C'

    f2 :: Maybe ((?a :: Bool) => Bool)
    f2 = Just ?a
}}}


{{{
$ ghc -c Bug.hs 

Bug.hs:5:15:
    Couldn't match expected type `(?a::Bool) => Char'
                with actual type `Char'
    In the first argument of `Just', namely 'C'
    In the expression: Just 'C'
    In an equation for `f1': f1 = Just 'C'

Bug.hs:8:15:
    Unbound implicit parameter (?a::(?a::Bool) => Bool)
      arising from a use of implicit parameter `?a'
    In the first argument of `Just', namely `?a'
    In the expression: Just ?a
    In an equation for `f2': f2 = Just ?a
}}}

I believe this used to work?",bug,new,normal,_|_,Compiler (Type checker),7.4.2,,,,Unknown/Multiple,Unknown/Multiple,GHC rejects valid program,Unknown,,,,