[jodrellblank]

One of his memorable blog posts was on generating permutations in array languages[1]. In his spoiler explanation he comments about generating permutations of 0..4 starting from permutations of 0..3 and a "magical matrix" to do that, and then discusses:

"What is the “best” perm function I can write in APL? This “best” is a benchmark not only on my own understanding but also on advancements in APL over the years." and then shows one he wrote in 1981, one from 1987, then "I worked on perm from time to time in this period [1990-2007] but in J rather than in APL", then one from 2008, and one from 2015, and where future improvements might be found. In a video talk on it he says "some of these sub-expressions can be improved, but the APL language cannot yet express the thought that I have"[2]. I just like that pattern of returning to the same problems and polishing both the language and solution over multiple decades.

[1] https://www.dyalog.com/blog/2015/07/permutations/

[2] https://youtu.be/e0rywC7-i0U?t=2254

[3] talk is taken from his writing here, I think: https://www.jsoftware.com/papers/50/