#4494: Another regression with type families
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Reporter: rl | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: GHC rejects valid
program
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Comment(by simonpj):
OK now I can reproduce it. Here's a cut down program.
{{{
{-# LANGUAGE TypeFamilies, RankNTypes, FlexibleContexts,
ScopedTypeVariables #-}
module T4494 where
type family H s
type family F v
bar :: (forall t. Maybe t -> a) -> H a -> Int
bar = error "urk"
call :: F Bool -> Int
call x = bar (\_ -> x) (undefined :: H (F Bool))
}}}
My compiler is build with DEBUG so it gives a more helpful error
{{{
T4494.hs:1:1:
TERRIBLE ERROR: double set of meta type variable
uf_ac3 := F Bool
Old value = (uf_ac3, a)
}}}
Great catch, thanks.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4494#comment:8>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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