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Additions to Control.Monad

    • Closed Issue created by olf @trac-olf

    I'd like to propose the following additions/changes to Control.Monad: mfilter can be generalised:

    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

    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:

    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:

    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.

    Trac metadata
    Trac field Value
    Version 7.8.2
    Type FeatureRequest
    TypeOfFailure OtherFailure
    Priority low
    Resolution Unresolved
    Component libraries/base
    Test case
    Differential revisions
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    Blocking
    CC ekmett, hvr
    Operating system
    Architecture
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