#5714: Add 'state' to the MonadState class
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Reporter: twanvl | Owner: ekmett@…
Type: task | Status: patch
Priority: normal | Milestone: 7.6.1
Component: libraries (other) | Version: 7.2.1
Keywords: mtl, monads-tf | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by ekmett):
Probably a good idea. A similar embedding can be defined for list, etc. if
we want to carry this through to its logical conclusion.
For reader, of course, the generalization of reader is 'asks', so the
question is which is the better name to put in the class?
One caveat with the above definition is that for the .Lazy versions
{{{
state :: (Monoid w, Monad m) => (s -> (a,s)) -> RWST r w s m a
state f = RWST $ \_ s -> let (a,s') = f a in return (a, s', mempty)
}}}
isn't as lazy as it could be with an extra ~ in the let.
These are a lot less ad hoc than they seem, since one can view them as the
rank 1 version of a higher rank hoisting operation which lifts monad
homomorphisms from n -> m into the monad transformer t n -> t m, where we
fix n to Identity, and use the canonical homomorphism from Identity -> m
given to us by return. The fully general version needs a rank 2 type
though, so its out of the question.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5714#comment:6>
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