Lest anyone think I'm a genius, I stole the Haskell code from here.

The "alternative solution" posted there doesn't suffer from the monomorphism restriction^*, and thus does less to propagate the myth that one has to be a genius to write code in Haskell. It also works correctly with empty input :-)

compress []       = []
compress [a]      = [a]           -- singleton list
compress (x:y:xs) = (if x == y then [] else [x]) ++ compress (y:xs)
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^*Something that, for the casual programmer, is quite technical but uninteresting, has a scary name, and a not-that-friendly error message.

And indeed, you can do that in Perl too:

multi compress () { () }
multi compress ($a) { item $a }
multi compress ($x, $y, *@xs) { $x xx ($x !=== $y), compress($y, |@xs)
+ }

my @x = <a a a a b c c a a d e e e e>;
say perl compress |@x;
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