#6032: HEAD (7.5.20120421) requires RankNTypes for a rank-2 type
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Reporter: dreixel | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.5
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Changes (by simonpj):
* difficulty: => Unknown
Comment:
The difficulty is this
* The argument of a rank-2 arrow type should have rank 1
* A vanilla H98 type sig has rank 1
* But technically `(Int -> forall a. a->a)` is rank 1, although it is not
H98.
GHC currently has the (somewhat ad-hoc) restriction that a rank-1 type
must have all its foralls at the front, which is why this program is
rejected. And yes that's a change in behaviour. (A different bug report
said that `(Int -> forall a. a->a)` was accepted despite no higher-rank
flags being on.
Perhaps it'd be better to always accept foralls to the right of an arrow.
After all, you need at least `-XForAllTypes` to be able to write them.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6032#comment:1>
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