Re: lpeg.Cb bug (?) with repeated evaluation of group capture pattern

[Sean Conner <sean@...>]

It was thus said that the Great Roberto Ierusalimschy once stated:
> > I found that for some reason LPeg is evaluating a pattern in a group
> > capture multiple times:
> > 
> > local patt = lpeg.Cg(lpeg.Ct"", "foo") * lpeg.Cb "foo" * lpeg.Cb "foo";
> > print(patt:match "") --> table: 0x1634280        table: 0x16342c0
> > 
> > (two unique tables!) Or:
> > 
> > local patt = lpeg.P {
> >     lpeg.Cg(lpeg.V "v", "foo") * lpeg.Cb "foo" * lpeg.Cb "foo";
> >     v = lpeg.Ct "" / function(...) print(...) return ... end,
> > }
> > print(patt:match "") --> table: 0x233d4a0 \n table: 0x22f0ab0 \n
> > table: 0x233d4a0        table: 0x22f0ab0
> > 
> > Whereas:
> > 
> > local patt = lpeg.Cg(lpeg.Cmt("", function(s,p) return p, {} end),
> > "foo") * lpeg.Cb "foo" * lpeg.Cb "foo";
> > print(patt:match "") --> table: 0x87a950 table: 0x87a950
> > 
> > (one table!)
> > 
> > Is this a bug?
> 
> Well, I know it works that way :-)
> 
> Except for 'Cmt', captures in LPeg have a functional flavor. LPeg does
> not specify in what order it evaluates its captures. It does not specify
> even whether it actually will execute a capture. All that matters are
> the results from the captures.  (For instance, the manual does not say
> that LPeg will not execute captures for failed match attempts; what it
> ensures is that those values are not used in its results.)
> 
> In general, we should avoid captures with side effects (again, except
> for 'Cmt') or, more to the point, we should not depend on these side
> effects.  (Checking whether two tables with the same contents are the
> same is an example of a dependency on a side effect.)  If we must
> accumulate something, we should use a fold.

  Then how do you explain the following:

	lpeg.Ct(lpeg.C(pattern)^0)

	lpet.Cg(lpeg.Cg(pattern,"name1") * lpeg.Cg(pattern,"name2"))

  Should we be using a fold for these as well?

  -spc