Re: Type Metatables for Table and Userdata - Powerpatch

[Sven Olsen <sven2718@...>]

> Unfortunately, it ("for k,v in t do ..." syntax) simultaneously obscures the function of the code.

I do not understand what you mean- could you elaborate on this?

Basically, while I also don't like the proliferation of "in pairs(...)" boilerplate, I have problems with the details of John's approach. Vanilla Lua's generic for syntax is already pretty complex, and under the self iterating objects patch the

  for <vars> in <explist> do

syntax gets significantly more complicated, because there's a switch on the type of the first result of <explist>. 

Consider:

  for k,v in t do

The programmer may think that this is simply code for walking through the table -- but the truth is that it's something considerably more complex, as t can legally be either a table or function iterator, and which way it's interpreted depends on it's raw type.  In the case that t is a table, there's 2 different metatable lookups that will impact the result (__iter and __pairs).

If all we really want is to reduce boilerplate pairs() calls, there's no need to hack the VM.  It's sufficient to just add a new transformation rule to the parser.

-Sven

PS: I'm still agonizing over candidate table iteration syntax.  At this point, I think my favorite is:

for k,v == t do 

but it appears that

for k,v = t do

would also work, as we could intercept the numeric for logic in the case that there's no stated upper limit.