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I'd like to add two instances to <tt>Data.Monoid</tt>, alongside of All/Any, Sum/Product, and First/Last.
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Here's a current instance (as a style example):
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<pre style="margin-left: 40px;" class="wiki">-- | Boolean monoid under conjunction.<br>newtype All = All { getAll :: Bool }<br> deriving (Eq, Ord, Read, Show, Bounded)<br><br>instance Monoid All where<br> mempty = All True
<br> All x `mappend` All y = All (x && y)<br></pre><p>
My proposed addition:
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<pre style="margin-left: 40px;" class="wiki">-- | Ordered monoid under 'max'.<br>newtype Max a = Max { getMax :: a }<br> deriving (Eq, Ord, Read, Show, Bounded)<br><br>instance (Ord a, Bounded a) => Monoid (Max a) where
<br> mempty = Max minBound<br> Max a `mappend` Max b = Max (a `max` b)<br><br>-- | Ordered monoid under 'min'.<br>newtype Min a = Min { getMin :: a }<br> deriving (Eq, Ord, Read, Show, Bounded)<br><br>instance (Ord a, Bounded a) => Monoid (Min a) where
<br> mempty = Min minBound<br> Min a `mappend` Min b = Min (a `min` b)<br></pre><br>I have a niggling uncertainty about the Ord & Bounded instances for Min a? Is there a reason flip the a ordering instead of preserving it?
<br><br>Suggested review period: two weeks (ending December 16).<br><br>Comments?<br><br> - Conal<br>