`sortOn` sorts a list by comparing the results of a key function applied to each element. `sortOn f` is equivalent to `sortBy . comparing f`, but has the performance advantage of only evaluating `f` once for each element in the input list. Historical note: This was already proposed in 2008 as part of http://www.haskell.org/pipermail/libraries/2008-October/010797.html It was, however, the recent re-attempt http://www.haskell.org/pipermail/libraries/2014-April/022489.html that let `sortOn` make it into base at last. Maybe the other functions mentioned in #2659 might be worth reconsidering as well.44512e3c
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