What does the "in" keyword in Lua do?

Question

I know it's usually combined with a for i,v in pairs() do loop (or ipairs, or even next) but what exactly is in?

Just to clarify, I know how to use it, I just don't know the logic behind it, how does it work/what does it return?

Answer 1

Lua's in is not a function or a variable. It's a part of the syntax for flow control. You can't replace it, you can't copy it, you can't even refer to it. It's rather like parentheses: a syntactic construct which has meaning for how a program is parsed, but which cannot be referred to within the program.

It doesn't "return" anything. It doesn't have "logic." It's more like a placeholder, or punctuation.

Answer 2

It doesn't do anything. It is syntax. It isn't a function. It isn't an opcode. It isn't a language feature. It is purely syntactical.

See the forlist function in lparser.c:

static void forlist (LexState *ls, TString *indexname) {
  /* forlist -> NAME {,NAME} IN explist1 forbody */
  FuncState *fs = ls->fs;
  expdesc e;
  int nvars = 0;
  int line;
  int base = fs->freereg;
  /* create control variables */
  new_localvarliteral(ls, "(for generator)", nvars++);
  new_localvarliteral(ls, "(for state)", nvars++);
  new_localvarliteral(ls, "(for control)", nvars++);
  /* create declared variables */
  new_localvar(ls, indexname, nvars++);
  while (testnext(ls, ','))
    new_localvar(ls, str_checkname(ls), nvars++);
  checknext(ls, TK_IN);
  line = ls->linenumber;
  adjust_assign(ls, 3, explist1(ls, &e), &e);
  luaK_checkstack(fs, 3);  /* extra space to call generator */
  forbody(ls, base, line, nvars - 3, 0);
}

Create the control variables. Handle the local variables in the comma list. Check that the next token is TK_IN which maps to luaX_tokens.