#7137: Unnecessary -XRank2Types requirement involving type alias containing
"forall" from another module
---------------------------------------+------------------------------------
Reporter: joeyadams | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: GHC rejects valid program | Testcase:
Blockedby: | Blocking:
Related: |
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The following builds with GHC 7.4, but not 7.6.
'''Bottom.hs'''
{{{
{-# LANGUAGE Rank2Types #-}
module Bottom where
type Bottom = forall a. a
data Pipe l i o u m r = Pipe
type GSource m o = forall l i u. Pipe l i o u m ()
}}}
'''Main.hs'''
{{{
import Bottom
myBottom :: Int -> Bottom
myBottom _ = error "Bottom"
main :: IO ()
main = myBottom 3
}}}
Note that Main.hs does not have the Rank2Types extension turned on.
myBottom does not have a rank-2 type. After expanding the type alias, you
get:
{{{
myBottom :: Int -> forall a. a
}}}
This is logically equivalent to:
{{{
myBottom :: forall a. Int -> a
}}}
GHC 7.4.2 accepts this use of type alias, but GHC 7.6 does not.
Note that if we make something that does in fact expand to a rank-2 type:
{{{
foo :: Bottom -> Int
foo x = x
}}}
GHC 7.4.2 correctly rejects the program unless you enable Rank2Types.
Also, if we use a standalone Bottom type alias:
{{{
myBottom :: Bottom
}}}
Then GHC 7.6 accepts the program. It does not accept Int -> Bottom
without Rank2Types, but it does accept Bottom by itself.
This issue is easy to work around: just add {-# LANGUAGE Rank2Types #-} or
{-# LANGUAGE RankNTypes #-} to the top of the module.
Is this a bug, or have the rules for type aliases and forall been
deliberately tightened?
P.S.: This ticket is for GHC 7.6.1-rc1. That version tag is not
available, though.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7137>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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