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    <p>Think about it: your code is saying "when x is in the remainder, prepend x to the result", i.e. creating a duplicate. You just need to change it to "when x is not in the remainder, prepend x to the result" and you get the correct function.</p> <p>This function differs from <code>Data.List.nub</code> in an important way: this function is more strict. Thus:</p> <pre><code>test [1..] = _|_ -- infinite loop (try it) nub [1..] = [1..] </code></pre> <p><code>nub</code> gives the answer correctly for infinite lists -- this means that it doesn't need the whole list to start computing results, and thus it is a nice player in the stream processing game.</p> <p>The reason it is strict is because <code>elem</code> is strict: it searches the whole list (presuming it doesn't find a match) before it returns a result. You could write that like this:</p> <pre><code>nub :: (Eq a) =&gt; [a] -&gt; [a] nub = go [] where go seen [] = [] go seen (x:xs) | x `elem` seen = go seen xs | otherwise = x : go (x:seen) xs </code></pre> <p>Notice how <code>seen</code> grows like the <em>output so far</em>, whereas yours grows like the <em>remainder of the output</em>. The former is always finite (starting at <code>[]</code> and adding one at a time), whereas the latter may be infinite (eg. <code>[1..]</code>). So this variant can yield elements more lazily.</p> <p>This would be faster (O(n log n) instead of O(n^2)) if you used a <code>Data.Set</code> instead of a list for <code>seen</code>. But it adds an <code>Ord</code> constraint.</p>
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    1. COI keep coming back to this response - as I continue with my attempts to learn Haskell, it is giving me deeper insights into the power of Haskell.
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