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J[1], ngn/k[2], oK[3], kona[4], and GNU APL[5] are the big oss interpreters. The main closed-source ones are Dyalog APL, Kx k, and Shakti k (which are still free for non-commercial use).

J. J is open-source.

Also, for K:

https://github.com/kevinlawler/kona https://bitbucket.org/ngn/k/src/master/

I'd also suggest looking at Pike's ivy as a simple example of an apl like system in go. https://github.com/robpike/ivy

I would argue that, being stylistically inconsistent with the way most apl is implemented, that's actually not such a great resource. I found 'whitney c' surprisingly approachable, with just a little patience; despite no prior experience with it, I was able to make an apl-ish interpreter in 7kb and a few hours. If you're curious about implementation details, that might be a more worthwhile exercise. Be sure to check out the parsing section[1] of the J dictionary.

1: https://www.jsoftware.com/help/dictionary/dicte.htm