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

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