How Do Ranks Work?

Question

The best way for me to understand J is emulating the interpreter. Since the language is compact and has little rules, it's been easy... with the exception of how ranks affect function evaluation.

I want to be able to see an expression and know what's J doing to get the result, step by step.

Is there a doc, or someone could give me an algorithm so I can calculate myself how a f " n m b is evaluated?

Thanks in advance.

Answer 1

For learning about Rank the most accessible text is probably chapter 6 of J for C Programmers. The section of Eric Iverson's Primer that begins with Atom and goes through Checkpoint E covers the topic more concisely. Chapter 7 of Learning J is another place Rank is covered. All are valuable.

The most in-depth examination of Rank is Roger Hui's essay Rank and Uniformity. Hui's paper will make better reading after you've studied the other texts on this topic. Should it come down to wanting the nitty-gritty of implementation, you could dive into the interpreter source code. Personally, I'd not do that last one. Were I wanting to look at implementation algorithms I'd build a little model, and check it against the results of a J interpreter to make sure that my understanding of Rank matches.

Rank, in my view, is the most important concept in J. It is quite abstract in that it applies across all the shapes that nouns can take. The associated concepts are important to learn. These include shape, frame, cell, and agreement. These are explained individually in the Primer, but they're explained in some manner every time the topic is dealt with in depth.

The better your understanding of the Rank conjunction, and the broader world of noun Rank and verb Rank in which it applies, the more useful you'll find the three sections of the Vocabulary that deal with this conjunction. (Those sections are m"n , u"n , and m"v u"v .)

If you do come to write any algorithms that help you examine things in a step-by-step fashion, other J programmers will enjoy seeing them, I'm sure. I don't know of anything along those lines other than the actual interpreter source code.