#4370: Bring back monad comprehensions
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Reporter: simonpj | Owner: nsch
Type: feature request | Status: new
Priority: normal | Milestone: 7.2.1
Component: Compiler | Version: 6.12.3
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Comment(by nsch):
That looks sweet, but how would you typecheck that? My naiv implementation
{{{
-- Type check 'mmap' with 'forall a b. (a -> b) -> m_ty a -> m_ty
b'
; mmap_op' <- tcSyntaxOp MCompOrigin mmap_op $
mkForAllTy alphaTyVar $ mkForAllTy betaTyVar $
(alphaTy `mkFunTy` betaTy)
`mkFunTy`
(m_ty `mkAppTy` alphaTy)
`mkFunTy`
(m_ty `mkAppTy` betaTy)
}}}
fails on compilation:
{{{
Couldn't match expected type `forall a b. (a -> b) -> [a] -> [b]'
with actual type `(a -> b) -> m a -> m b'
In a stmt of a monad comprehension: then group by x using groupWith
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4370#comment:33>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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