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{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
FlexibleInstances, UndecidableInstances #-}
-- UndecidableInstances now needed because the Coverage Condition fails
module ShouldFail where
-- A stripped down functional-dependency
-- example that causes GHC 4.08.1 to crash with:
-- "basicTypes/Var.lhs:194: Non-exhaustive patterns in function readMutTyVar"
-- Reported by Thomas Hallgren Nov 00
-- July 07: I'm changing this from "should fail" to "should succeed"
-- See Note [Important subtlety in oclose] in FunDeps
primDup :: Int -> IO Int
primDup = undefined
dup () = call primDup
-- call :: Call c h => c -> h
--
-- call primDup :: {Call (Int -> IO Int) h} => h with
-- Using the instance decl gives
-- call primDup :: {Call (IO Int) h'} => Int -> h'
-- The functional dependency means that h must be constant
-- Hence program is rejected because it can't find an instance
-- for {Call (IO Int) h'}
class Call c h | c -> h where
call :: c -> h
instance Call c h => Call (Int->c) (Int->h) where
call f = call . f
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