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

[Roberto Ierusalimschy <roberto@...>]

> 	[...]
> 
>   So the first Cb() returns the table returned by Ct().  Why does the second
> call to Cb() return a new table?  Cg() groups all values returned by the
> pattern into a single capture, so there should only *be* one capture.  Or am
> I misreading the manual?  Does it mysteriously "rerun" the pattern each time
> it's called?

The manual does not say *when* (or even *if*) a capture 'Ct' runs. It
only says that, *when* it runs, it returns a table with such and such
elements. (Probably the manual would be clearer if it says that the
capture "returns a table" instead of "creates a table".) So, nothing
prevents it from running several times (therefore, creating several
tables) or not running at all, if its value is not needed.


>   And now the two calls to Cb() return the same table.  I think the
> confusion here is how
> 
> 	Cg(Ct"","foo")
> 
> seems to keep returning new captures for the given pattern for each call to
> Cb(), whereas
> 
> 	Cg(Cmt("",function(s,p,c) return p,{} end),"foo")
> 
> returns the same capture for each call to Cb().  What, exactly, is the
> distinction between the two?

The manual says about 'Cmt':

   Unlike all other captures, this one is evaluated immediately when a
   match occurs. It forces the immediate evaluation of all its nested
   captures and then calls function.

This is the only capture that specifies *when* (and *if*) it runs. It
is the only one where you can count on side effects.

-- Roberto