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copied!<p>It's always tricky composing two-argument functions. Here's an implementation:</p> <pre><code>invert :: Ordering -> Ordering invert GT = LT invert LT = GT invert EQ = EQ sosort :: (Ord a, Ord b) => [(a, b)] -> [(a, b)] sosort = sortBy (\p p' -> invert $ uncurry compare $ double fst p p') . sortBy (\p p' -> uncurry compare $ double snd p p') where double f a a' = (f a, f a') </code></pre> <p>Because <code>sortBy</code> expects a function of two arguments, function composition isn't so nice.</p> <p>I have tested this code, and it works on your example.</p> <p>As Fred points out, you can write <code>compare EQ</code> instead of <code>invert</code>. As Dario points out, I could be using <code>on</code> from <code>Data.Function</code>, but in fact <code>on compare == comparing</code>, which I can use instead. Now the code can be read only by a Haskell Master:</p> <pre><code>sosort :: (Ord a, Ord b) => [(a, b)] -> [(a, b)] sosort = sortBy (compare EQ `post` comparing fst) . sortBy (comparing snd) where post f g x x' = f (g x x') </code></pre> <p>I have compiled and run this code and it works on the original example.</p> <p>(I haven't got any votes for this answer, but thanks to good comments, I sure have learned a lot about the Haskell library. Who knows what function <code>post</code> is equivalent to? Not Hoogle...)</p> <p>It would be <strong>more idiomatic to write a suitable comparison function for pairs</strong>, but your question asked for consecutive sorts.</p>

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