> Would it be possible to use Futhark to rewrite the APL implementation instead of the Haskell one? That would make an interesting comparison.

Sadly, from what I can see, the APL version makes use of so-called nested arrays in the 'words' function, specifically arrays of strings (this is different from multidimensional arrays). Futhark does not directly support nested arrays. A rewrite of the APL implementation would require using a quite different algorithm (or a nontrivial encoding).

But my APL is a bit rusty, so I may be wrong.