Re: [boost] Re: [ANN] Spirit C++ parser framework.

From: Larry Evans (jcampbell3_at_[hidden])
Date: 2001-05-26 20:00:46

joel de guzman wrote:

> From: "Douglas Gregor" :
>
> > > I agree.  I think that reads much more clear to me.  The [][] idea is
> > > only useful if you REALLY want to use []'s.
> >
> > I also like the a.repeat(0, more) and repeat<0, more>(a) syntax, though my
> > preference would be to remove some of the extra typing and go to:
> >
> > a<0, more>()
> > a(0, more)
> >
> > Supporting both may help some optimizations but also allows variable
> lengths.
> >
>
> Yet after sometime tinkering with the [][], it grows on you. One thing
> I like about the [] is that it clearly separates iteration from grouping:
>
> (a | b | c)[0][more]
>
> vs.
>
> (a | b | c)(0, more)
>
> So, why is the [][] yucky?
>

With normal bnf, for any expression, e.

     e** == e*

the analogy with [][] would be
     e[0][more][0][more] == e[0][more]
but then what does:
      e[0]
mean?  The empty language or the empty string?
And is the following:
      e[0] == e[0][more][0]
true?  Or are an even number of [] required?  In other words,
   e[whatever]  | f
   e[whatever] >> f
are not defined?