#5596: "f c = a $ b c", "f = a . b" does not.
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Reporter: guest | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.2.1
Resolution: invalid | Keywords:
Testcase: | Blockedby:
Difficulty: | Os: Linux
Blocking: | Architecture: x86_64 (amd64)
Failure: GHC rejects valid program |
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Changes (by simonpj):
* status: new => closed
* resolution: => invalid
Comment:
Yes, this is bad, but it's because GHC is trying to help!
Fundamentally, neither `(runST $ f)` nor `(runST . f)` should work,
because both require calling `($)` or `$(.)` respectively ''at a
polymorpic type''; that is, they both require impredicative polymoprhism.
(In earlier versions of GHC Dimitrios and I implemented our
[http://research.microsoft.com/%7Esimonpj/papers/boxy/ boxy-type paper],
but the implementation was grotesquely complicated and unpredicatable, so
we rolled it back.)
However, the idiom `(runST $ do ...blah...)` had become very common in
practice, so I added a TOTALLY AD-HOC special case typing rule, for terms
of the form `(e1 $ e2)`. In this special se there is no problem with
impredicativity, so away we go. And that's why the two differ: there is
no special case for `(.)`.
I could compound the problem by adding a special typing rule for `(e1 .
e2)` as well, but I'm not sure that would really be an improvement.
The special typing rule for `($)` isn't even documented in the manual,
because I didn't really want to encourage people to exploit it. It's a
secret feature.
Cf #4295.
I'll close this as invalid, but acknowledging that things are not great.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5596#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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