#3155: TypeOperators/Rank2Types clash
----------------------------------+-----------------------------------------
Reporter: MartijnVanSteenbergen | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.10.1 | Severity: normal
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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Consider the following piece of code:
{{{
{-# LANGUAGE GADTs #-}
data Any s where
Any :: s ix -> ix -> Any s
data AnyR s r where
AnyR :: s ix -> r ix -> AnyR s r
unR :: (forall ix. r ix -> ix) -> AnyR s r -> Any s
unR f (AnyR ix rix) = Any ix (f rix)
}}}
GHC reports:
{{{
Illegal operator `.' in type `forall ix . (r ix -> ix)'
Perhaps you intended to use -XRankNTypes or similar flag
to enable explicit-forall syntax: forall <tvs>. <type>
}}}
Enabling TypeOperators causes the error to change to:
{{{
Occurs check: cannot construct the infinite type: ix = r ix
In the pattern: AnyR ix rix
In the definition of `unR': unR f (AnyR ix rix) = Any ix (f rix)
}}}
Which is very confusing, as the solution (still) is to turn on Rank2Types.
Could it be made so that the error message stays the same when
TypeOperators is turned on?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3155>
GHC <http://www.haskell.org/ghc/>
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