#4370: Bring back monad comprehensions
------------------------------+---------------------------------------------
Reporter: simonpj | Owner:
Type: feature request | Status: new
Priority: normal | Milestone: _|_
Component: Compiler | Version: 6.12.3
Resolution: | Keywords:
Testcase: | Blockedby:
Difficulty: | Os: Unknown/Multiple
Blocking: | Architecture: Unknown/Multiple
Failure: None/Unknown |
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Changes (by simonpj):
* owner: simonpj =>
* priority: high => normal
* status: patch => new
* milestone: 7.2.1 => _|_
Comment:
George, concerning your question, the trouble is that (as the user manual
says) we check that the `fmap` function in scope has type
{{{
fmap :: forall a b. (a->b) -> n a -> n b
}}}
Why those foralls? Because we are going to apply `fmap` to lots of
different arguments. See Section 4.4 of http://research.microsoft.com/en-
us/um/people/simonpj/papers/list-comp/list-comp.pdf for a discussion of
why we ask for polymorphism.
So I don't know how to give you what you want here... but I can also see
your problem.
In short, its not just an implementation bug; there's something
substantial to think about here. I'll re-open the ticket, to keep track
of the question, but I'm not sure how to proceed.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4370#comment:65>
GHC <http://www.haskell.org/ghc/>
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