Opened 2 years ago

Closed 14 months ago

#9385 closed feature request (invalid)

Additions to Control.Monad

Reported by: olf Owned by: ekmett
Priority: low Milestone:
Component: Core Libraries Version: 7.8.2
Keywords: report-impact Cc: hvr, ekmett, core-libraries-committee@…
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown Test Case:
Blocked By: Blocking:
Related Tickets: Differential Rev(s):
Wiki Page:


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


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.

Change History (5)

comment:1 Changed 2 years ago by ekmett

This sort of proposal should go through the libraries@… mailing list rather than the GHC Trac.

Please feel free to submit this for discussion there to invite wider discussion.

comment:2 Changed 2 years ago by thoughtpolice

  • Component changed from libraries/base to Core Libraries
  • Owner set to ekmett

Moving over to new owning component 'Core Libraries'.

comment:3 Changed 23 months ago by thomie

  • Cc core-libraries-committee@… added
  • Status changed from new to infoneeded

olf: did you submit any proposal to the libraries mailinglist for this?

See also:

comment:4 Changed 20 months ago by hvr

  • Keywords report-impact added

comment:5 Changed 14 months ago by thomie

  • Resolution set to invalid
  • Status changed from infoneeded to closed

Please reopen with a mandate from the libraries@ list.

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