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<p>Aren't Min and Max sufficient by themselves:<br>
<br>
>>> foldMap (Just . Min &&& Just . Max)
[1..10 :: Integer]<br>
(Just (Min {getMin = 1}),Just (Max {getMax = 10}))<br>
</p>
<p>- Oleg<br>
</p>
<div class="moz-cite-prefix">On 10.11.2020 23.19, Jon Purdy wrote:<br>
</div>
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cite="mid:CAE3FWBv7JSyj8LMG0C+dbXnrYzO_oWWBHyDjpYYRi79vSwYT7A@mail.gmail.com">
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<div>The ‘Min’ and ‘Max’ newtypes from ‘Data.Semigroup’
require a ‘Bounded’ constraint for their ‘Monoid’
instances, because they wrap a value of type ‘a’ rather
than ‘Maybe a’. The latter is something I’ve needed a few
times lately for types that cannot be bounded (or just
don’t want to be), and it seems base-worthy to me.<br>
<br>
So if it hasn’t been proposed and dismissed already (or if
it has been proposed, but just got backburnered), I’d like
to suggest adding the following new types, and at least
the following instances, to either ‘Data.Semigroup’ or
‘Data.Monoid’:<br>
<br>
</div>
newtype Minimum a = Minimum { getMinimum :: Maybe a }<br>
</div>
<div> deriving (Eq, Ord, Monoid, Semigroup)<br>
</div>
<div> via (Down (Maybe (Down (Max a))))<br>
</div>
<div><br>
</div>
newtype Maximum a = Maximum { getMaximum :: Maybe a }<br>
</div>
<div> deriving (Eq, Ord, Monoid, Semigroup)<br>
</div>
<div> via (Maybe (Max a))<br>
</div>
<div><br>
</div>
<div>These instances could be specified explicitly instead of
with ‘DerivingVia’, but the essential notion here is that
‘Maximum’ uses its empty value ‘Nothing’ as a “negative
infinity”, less than all other ‘Just’ values, while ‘Minimum’
reverses that to use ‘Nothing’ as the corresponding “positive
infinity”.<br>
<br>
</div>
<div>I assume these would also want more or less the same array
of instances as for the existing ‘Min’ and ‘Max’, and there is
the usual opportunity to bikeshed about the naming and how
these instances ought to relate to one another. I figure that
at least the ‘Ord’ instance should match the ordering that the
‘Semigroup’ instance uses, to avoid some potential for
confusion/bugs.<br>
<br>
</div>
<div>As for use cases, I tend to reach for these things either
in configuration records with many fields that I’m combining
by semigroup append (Max, Any, set union, and so on) or when I
have a possibly-empty container and don’t want to write e.g.
‘maximum (0 : xs)’ to avoid partiality. For example, ‘foldMap
(Minimum . Just &&& Maximum . Just)’ computes the
minimum and maximum in a single pass, returning Nothing for a
null input.</div>
<div><br>
</div>
<div>Any thoughts, critiques, questions, rebuttals, edge cases,
use cases, &c. are most welcome!<br>
<br>
</div>
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<br>
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