Question

In Lua, using the = operator without an l-value seems to be equivalent to a print(r-value), here are a few examples run in the Lua standalone interpreter:

> = a nil > a = 8 > = a 8 > = 'hello' hello > = print function: 003657C8 

And so on…

My question is : where can I find a detailed description of this use for the = operator? How does it work? Is it by implying a special default l-value? I guess the root of my problem is that I have no clue what to type in Google to find info about it 🙂

edit:

Thanks for the answers, you are right it’s a feature of the interpreter. Silly question, for I don’t know which reason I completely overlooked the obvious. I should avoid posting before the morning coffee 🙂 For completeness, here is the code dealing with this in the interpreter:

while ((status = loadline(L)) != -1) {   if (status == 0) status = docall(L, 0, 0);   report(L, status);   if (status == 0 && lua_gettop(L) > 0) {  /* any result to print? */     lua_getglobal(L, 'print');     lua_insert(L, 1);     if (lua_pcall(L, lua_gettop(L)-1, 0, 0) != 0)       l_message(progname, lua_pushfstring(L,                            'error calling ' LUA_QL('print') ' (%s)',                            lua_tostring(L, -1)));   } } 

edit2:

To be really complete, the whole trick about pushing values on the stack is in the ‘pushline’ function:

if (firstline && b[0] == '=')  /* first line starts with `=' ? */   lua_pushfstring(L, 'return %s', b+1);  /* change it to `return' */ 

Answer 1

Quoting the man page:

In interactive mode … If a line starts with ‘=’, then lua displays the values of all the expressions in the remainder of the line. The expressions must be separated by commas.