Proposal: Add Data.Monoid.mwhen :: Monoid m => Bool -> m -> m

[James Ashwell]

This would mimic the behaviour of Control.Monad.when for monoids rather than applicatives.
mwhen :: Monoid m => Bool -> m -> m
mwhen b a
 | b         = a
 | otherwise = mempty

-- Examples:
when :: Applicative f => Bool -> f () -> f ()
when b = getAp . mwhen b . Ap
-- mwhen b = getConst . when b . Const

guard :: Alternative f => Bool -> f ()
guard b = getAlt (mwhen b (pure ()))

applyIf :: Bool -> (a -> a) -> (a -> a)
applyIf b = appEndo . mwhen b . Endo

-- Using Monoid m => Monoid (a -> m)
-- mwhen b f x   = if b then f x   else mempty = mwhen b (f x)
-- mwhen b f x y = if b then f x y else mempty = mwhen b (f x y)
-- etc

-- mwhen b x <> y = if b then x <> y else y
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