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--></style></head><body lang=EN-GB link=blue vlink="#954F72"><div class=WordSection1><p class=MsoNormal>This would mimic the behaviour of <span style='font-family:"Courier New"'>Control.Monad.when</span> for monoids rather than applicatives.<o:p></o:p></p><p class=MsoNormal><span style='font-family:"Courier New"'>mwhen :: Monoid m => Bool -> m -> m<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>mwhen b a<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'> | b = a<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'> | otherwise = mempty<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>-- </span>Examples:<span style='font-family:"Courier New"'><o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>when :: Applicative f => Bool -> f () -> f ()<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>when b = getAp . mwhen b . Ap<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>-- mwhen b = getConst . when b . Const<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>guard :: Alternative f => Bool -> f ()<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>guard b = getAlt (mwhen b (pure ()))<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>applyIf :: Bool -> (a -> a) -> (a -> a)<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>applyIf b = appEndo . mwhen b . Endo<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>-- </span>Using<span style='font-family:"Courier New"'> Monoid m => Monoid (a -> m)<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>-- mwhen b f x = if b then f x else mempty = mwhen b (f x)<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>-- mwhen b f x y = if b then f x y else mempty = mwhen b (f x y)<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>-- </span>etc<o:p></o:p></p><p class=MsoNormal><span style='font-family:"Courier New"'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-family:"Courier New"'>-- mwhen b x <> y = if b then x <> y else y<o:p></o:p></span></p></div></body></html>