Re: Proposal: Add Data.Semigroup to base, as a superclass of Monoid

11 Jun 2013


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John Wiegley  writes:
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4. (Optional, recommended) That we fix the Monoid instance for Maybe to be:
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instance Semigroup a => Semigroup (Maybe a) where
         Just x <> Just y = Just (x <> y)
         _ <> _ = Nothing
Slight correction (thanks to Brent Yorgey for catching this):

    instance Semigroup a => Semigroup (Maybe a) where
         Just x <> Just y = Just (x <> y)
         Nothing <> x = x
         x <> Nothing = x

-- 
John Wiegley
FP Complete                         Haskell tools, training and consulting
http://fpcomplete.com               johnw on #haskell/irc.freenode.net