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Comment by klibertp | Hacker News Reader

I'd like to be awed and amazed, but the frightening thought of encountering such a thing in production and under tight deadline kind of doesn't let me :)

EDIT: I've edited the code a bit so that it doesn't look like PERL or J itself https://gist.github.com/piotrklibert/4d32c8cc6fcf20643a257a2...

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J's commercially aligned cousin, K, is routinely encountered in financial/ trading backends. Part of job security, I presume.

I know some K, but I am not a particular fan of it. I admire the efficiency, though.

I got as far as trying to build and run it, but bugged out, thusly:

    $ ./one-page-j
    warning: this program uses gets(), which is unsafe.

    Segmentation fault: 11

Ah well, its still cute to try to read and understand ..

The code was written on paper. Without the usual compile/debug cycles during development.

The amazing part is how little cleanup work is required to make the code work. It's an entire programming language after all!

I got it to compile but what is it even supposed to do?

Should start a REPL. The `main` function looks like this:

  main(){
    C s[99];
    while(gets(s)) 
      pr(ex(wd(s)));
  }

which is a very direct expression of read-execute-print-loop.

It's a tiny interpreter. You can create vectors, add them, concatenate them, store them to variables, count their lengths, and so on. An example session:

    x=~5
    5
    0 1 2 3 4
    y=x,x
    10
    0 1 2 3 4 0 1 2 3 4
    x+y
    10
    0 2 4 6 8 0 1 2 3 4

To understand the source, the key are the arrays `C vt[]`, `A(vd[])()`, and `A(vm[])()`, which maps the verb token to the dyads and monads respectively. `V1` functions are monads, `V2` are dyads.

It's not 64-bit clean. If you compile it as 32-bit it will run.

A really helpful edit would be the V1 and V2 macro expansions; then it's fairly straightforward C with an odd naming scheme.

I don't think so. It's easier to see if something is monadic or dyadic with the macros, and you don't need to parse the function head to know what args and of what types it takes.

> monadic or dyadic

Also known as unary and binary, TIL. They're arity, not FP monads.

Adicity is an alternate naming scheme for arity, based on Greek rather than Latin roots.

Also a common naming scheme in music, which is where APL got the terms from: one note is a monad, two simultaneously is a dyad, three is a triad, four is a tetrad, etc.

Fwiw APL's use of monad in this sense predates the FP use of the term by several decades.

Monads, in the FP sense, come from the same construction in Category Theory. According to Wikipedia, this was first "invented" in 1958, but the term "monad" was coined later by Saunders Mac Lane.

You could also run the preprocessor to make it clearer.

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