#5744: List layouts
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Reporter: nsch | Owner: nsch
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.2.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Changes (by NeilMitchell):
* cc: ndmitchell@… (added)
Comment:
Note that you can already use {{{do}}} to define a {{{Monoid}}}:
{{{
data Wrap m a = Wrap m a
unwrap :: Wrap m a -> m
unwrap (Wrap m a) = m
wrap :: m -> Wrap m ()
wrap m = Wrap m ()
instance Monoid m => Monad (Wrap m) where
return = Wrap mempty
Wrap m1 a1 >>= f = Wrap (m1 `mappend` m2) a2
where Wrap m2 a2 = f a1
foo :: [Int]
foo = unwrap $ do
wrap [1,2,3]
wrap [4,5]
wrap [6]
}}}
If you sugar up a particular monad and add some nicer types and helper
functions, it can end up looking quite pleasant. I use this trick in the
Shake library, so rules can be specified using either do notation or as a
monoid, even though they are really a monoid.
(Personally, I agree with Simon Marlow, that it's probably not a problem
that needs solving - but hopefully the above insight might solve some of
the issues)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5744#comment:15>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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