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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
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CC	ekmett, hvr
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