Re: functioncall vs varlist ambiguity

[Roberto Ierusalimschy <roberto@...>]

> I'm writing yet another parser for Lua (more background soon,
> hopefully) and I'm finally struggling with the ambiguity between
> varlists (in assignments) and functioncalls.  If I'm not mistaken,
> this ambiguity ensures that Lua is not LL(k) for any k.

I think this ambiguity has nothing to do with varlists or Lua in
particular.  It happens to any language that allows a minimum of
flexibility in what is a variable and that allows variables to refer to
functions.

When the parser starts reading something like "a.f[x].g[i][k].f" it
cannot know whether this is a function call or an assignment, no
matter how much look-ahead it uses.


> Is there a clever trick to deal with this?

Use a bottom-up parser ;) They are much more powerful than LL parsers.

Or use the (dirty) trick that the Lua parser uses. Twist the
grammar to something more or less like this:

  callOrAssig = prefixexp rest
  rest = "," restassign | "=" restassign1 | "(" etc.

Things like "f(x) = 3" must be rejected outside the parser.

-- Roberto