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Comment by stiff | Hacker News Reader

Once you get past the formatting and cryptic variable names it's actually a pretty straightforward program, have a look at one of the older threads if you are interested in understanding it:

https://news.ycombinator.com/item?id=8533843

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Unfortunately, I tried getting it to run and it simply doesn't work. Unless the expected behavior of every input is to crash the interpreter.

EDIT: In fact, you can see for yourself using https://gist.github.com/piotrklibert/4d32c8cc6fcf20643a257a2... (thanks, klibertp!)

  $ ./j
  warning: this program uses gets(), which is unsafe.
  5 + 10
  Segmentation fault: 11
  $ ./j
  warning: this program uses gets(), which is unsafe.
  5 + 10 20 30
  j(70679,0x7fffa46de3c0) malloc: *** error for object 0x7f7fc74001b8: incorrect checksum for freed object - object was probably modified after being freed.
  *** set a breakpoint in malloc_error_break to debug
  Abort trap: 6

From http://code.jsoftware.com/wiki/Studio/TasteofJPart1, `5 + 10` should be a valid J program, but it crashes. Is there any input that doesn't crash it?

I did run it back in the day, it works, I think it just happens to assume a 32 bit OS. You can fix the gist by changing this:

  I *ma(n){ R (I*) malloc(n*4); }

to:

  I *ma(n){ R (I*) malloc(n*8); }

Note that it's a very tiny interpreter for a very tiny simple language - it does not have any error handling (it still WILL segfault if you enter "incorrect" programs, where "correct" is whatever doesn't crash the interpreter :)), the letters a-z are the only available variable names etc., this is why it is so short. Once you get over the APLish way of writing the C code, and once you deduce what the operators are intended to do, it's a pretty straightforward C program.

Here is a sample session:

This piece of the gist is the "symbol table" for respectively binary and unary operators, you can use it to get started understanding the language:

  C vt[] = "+{~<#,";
  A(*vd[])() = { 0, plus, from, find, 0, rsh, cat},
   (*vm[])() = { 0, id, size, iota, box, sha, 0};

When I last looked at this I found:

plus a+b: addition

  Example: 1+2 -> 3

from a{b: selects the a-th element from b

  Example: 3{4,5,6,7,8 -> 7

find a~b: Not implemented? Or maybe this exploits some old-style C behavior?

rsh a#b: repeat a items of b

  Example: 3#9 -> [9,9,9]

  Example: 7#2,3 -> [2,3,2,3,2,3,2]

cat a,b: concatenate item a onto the list b

  Example: 1,2,3 -> [1,2,3]

id +a: identity

  Example: +7 -> 7

size {a: get the size (in the first dimension) of a

  Example: {7,8,9 -> 3

  Example: a=4,5
           b=a#7
           {b -> 4

iota ~a: enumerates all integers from 0 to a-1

  Example: ~3 -> 0,1,2

  Example: ~2,3 -> [0,1,2,3,4,5]

box <a: Put a into an indirection box

  Example: <1 -> <1

sha #a: get the shape (size in all dimensions) of a

  Example: #7,8,9 -> 3

  Example: a=4,5
           b=a#7
           #b -> 4,5

I don't remember or haven't figured out how to dereference a boxed value, or what this can be used for.

The struct A has members:

  t: The "type": 0=integer, 1=boxed reference
  r: The "rank": size of array d
  d: The "dimensions": Multiply to get the size of array p
  p: The actual data. Can be integers or pointers to boxed data (cast to a 32-bit integer!)

Wow. This is genuinely incredible. The interpreter works, and all of your examples run.

Thank you for detailing all of this.

It works! I compiled it with `gcc j.c -m32 -o j` and it runs now. Awesome.

Thanks for the helpful example program.

EDIT: Sort of. `5+10` crashes, as does `a=5 \n b=10`. But single digit numbers work, so this is pretty cool.

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