#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: -------------------------------------+------------------------------------- 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. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9385 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler