[GHC] #9385: Additions to Control.Monad

30 Jul 2014


      #9385: Additions to Control.Monad
-------------------------------------+-------------------------------------
       Reporter:  olf                |                   Owner:
           Type:  feature request    |                  Status:  new
       Priority:  low                |               Milestone:
      Component:  libraries/base     |                 Version:  7.8.2
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 I'd like to propose the following additions/changes to Control.Monad:
 `mfilter` can be generalised:
 {{{#!hs
 gen_mfilter :: Monad m => (a -> m ()) -> m a -> m a
 gen_mfilter f ma = (\a -> liftM (const a) (f a)) =<< ma
 }}}
 Now we obtain the old `mfilter` as
 {{{#!hs
 gen_mfilter.(guard.)
 }}}
 Further, `m ()` is a monoid for every monad, which would cause conflicts
 for `[()]`, to name one example. (The usual monoid instance of `[()]` is
 addition of natural numbers, while the monadic monoid instance is
 multiplication.) More generally, the monoid `m ()` acts on every type `m
 a` in the following way:
 {{{#!hs
 mtimes :: Monad m => m () -> m a -> m a
 mtimes = gen_mfilter.const = liftM2 (flip const)
 when = mtimes.guard
 }}}
 For example, each element of a list can be duplicated like this:
 {{{#!hs
 mtimes [(),()]
 }}}
 To see why these functions are useful, consider the `DDist` monad of the
 ProbabilityMonads package: Since `DDist ()` is essentially the monoid of
 real numbers with multiplication, `gen_mfilter f` updates a distribution
 by multiplying the weight of `x` by `f x`.
 Another example is the state monad `ST s`, where type `(a -> ST s ())` is
 essentially `a -> s -> s`, so these functions encode changes that, when
 used with `gen_mfilter` alter state, not the value.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9385
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