sortOn sorts the specified
Seq by
comparing the results of a key function applied to each element. The
sort is stable, meaning the order of equal elements is preserved.
sortOn f is equivalent to
sortBy
(compare `on` f), but has the performance advantage
of only evaluating
f once for each element in the input
Seq.
An example of using
sortOn might be to sort a
Seq of
strings according to their length:
sortOn length (fromList ["alligator", "monkey", "zebra"]) == fromList ["zebra", "monkey", "alligator"]
If, instead,
sortBy had been used,
length would be
evaluated on every comparison, giving <math> evaluations, rather
than <math>.
If
f is very cheap (for example a record selector, or
fst),
sortBy (compare `on` f)
will be faster than
sortOn f.