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copied!<p>There are two problems I think might be troubling you.</p> <p>First of all, loading a file with your definitions into ghci, I try</p> <pre><code>ghci> csort (>= x) [2,1,3] <interactive>:1:10: Not in scope: 'x' </code></pre> <p>The way you wrote it, <code>f</code> takes two parameters. And indeed <code>x</code> is not in scope here. So the custom filter function should be simply <code>(>=)</code>.</p> <pre><code>ghci> csort (>=) [2,1,3] ***Exception: blahblah: Non-exhaustive patterns in function csort </code></pre> <p>Now the real problem: non-exhaustive patterns. What does this mean? You've written a definition of how to sort a list with at least one element. But how do you sort a list with no elements? Simple, you ignore the custom filter function, and simply produce an empty list. Since an empty list has no elements, it is already "sorted".</p> <pre><code>csort _ [] = [] </code></pre> <p>Once I added that line to the source file, it suddenly worked. The pattern <code>[]</code> compliments the pattern <code>(x:xs)</code>, and those two patterns, together, are exhaustive (a list is either empty, or it has at least one element).</p> <pre><code>ghci> csort (>=) [2,1,3] [1,2,3] ghci> csort (<) [2,1,3] [3,2,1] ghci> quickCheck (\xs -> csort (<) xs == (reverse $ csort (>) xs)) +++ OK, passed 100 tests. </code></pre> <hr> <p>Here's my sort.hs file:</p> <pre><code>csort _ [] = [] csort f (x:xs) = csort f lesser ++ [x] ++ csort f greater where lesser = [e | e <- xs, f x e] greater = [e | e <- xs, not $ f x e] </code></pre> <p>I have no idea why you would still have errors; this works perfectly well for me.</p>

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