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    <p>If you want to do it in a single <code>foldr</code>, we can exploit the fact that <code>Ord a =&gt; Ord (Maybe a)</code>, that is, any ordering on <code>a</code> can be extended to an ordering on <code>Maybe a</code>.</p> <p>We also have <code>Just x &gt; Nothing</code>, for all <code>x :: Ord a =&gt; a</code>.</p> <pre><code>mymax :: (Ord a) =&gt; [a] -&gt; Maybe a mymax = foldr (\x b -&gt; let x' = Just x in if x' &gt; b then x' else b) Nothing -- equivalently mymax = foldr (\x b -&gt; let x' = Just x in max x' b) Nothing mymax = foldr (\x' b -&gt; max x' b) Nothing . map (\x -&gt; Just x) mymax = foldr max Nothing . map Just </code></pre> <p>If we want to do minimum, we'll have to do it a bit differently, since <code>Nothing</code> is the lower bound for the type <code>Ord a =&gt; Maybe a</code>, which means that <code>foldr min Nothing . map Just == const Nothing</code>, which isn't useful. </p> <pre><code>mymin :: (Ord a) =&gt; [a] -&gt; Maybe a mymin = foldr (\x b -&gt; case b of Nothing -&gt; Just x Just y -&gt; Just (min x y) ) Nothing -- which is equivalent to mymin = foldr (\x b -&gt; Just $ case b of Nothing -&gt; x Just y -&gt; min x y ) Nothing mymin = foldr (\x b -&gt; Just $ maybe x (min x) b) Nothing mymin = foldr (\x -&gt; Just . maybe x (min x)) Nothing </code></pre> <p>Honestly, though, I think pattern matching makes solutions a lot clearer</p> <pre><code>mymax [] = Nothing mymax (a:as) = Just $ foldr max a as mymin [] = Nothing mymin (a:as) = Just $ foldr min a as </code></pre>
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    1. COMy only question is what is the notation "$", I tried googling it, but I'm getting a lot of results about buying haskell books...
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    2. CO@ZAX maybe you should buy a haskell book then :) joke aside, `$` is a standard application function. `f $ x` is the same as `f x`. It is typically used to eliminate parenthesis. `Just $ foldr max a as` == `Just (foldr max a as)`. Without `$` this quickly gets more complicated with lots more parenthesis to untangle when reading the code. For example, try to place the closing parenthesis in the statement `Just (case b of .....)`, and then try to figure out how to read the code - how to match the ). `$` tells the reader "...and the rest of the expression is the argument of the previous function".
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    3. COAlso, if you want to look up haskell operators try [hoogle](http://www.haskell.org/hoogle/) and [hayoo](http://holumbus.fh-wedel.de/hayoo/hayoo.html)
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