Re: [boost] [Spirit/Qi] Sequential-or attribute access in semantic action

Subject: Re: [boost] [Spirit/Qi] Sequential-or attribute access in semantic action
From: Hartmut Kaiser (hartmut.kaiser_at_[hidden])
Date: 2009-12-04 13:41:21

> > Well, I expect it to be an optional<S> only as well, but I'm not
> sure.
> > Normally this kind of redundancy gets collapsed away (and there is no
> > reason for it to be an optional<optional<S>>.)
> >
> > Ok, I looked: you're right, but I consider this to be a bug. This
> will
> > be fixed in the next release, then.
> >
> Oh right.  I had considered it desired behaviour.  The grammar spec is
> 
>     (start || (':' >> -start))
> 
> and not
> 
>     (start || (':' >> start))
> 
> I would expect optional<S> for both only in the latter case.
> 
> In the former I am deliberately allowing the specifier to the right of
> the colon to be omitted -- within the already optional right-hand-side
> of the sequential-or.  This allows for input such as
> 
>  (1)   $x       // $x (assume it references a sequence)
>  (2)   $x[7]    // element 7 of $x
>  (3)   $x[7:]   // a slice of $x beginning at element 7
>  (4)   $x[:7]   // a slice of $x up to element 7
>  (5)   $x[2:7]  // a slice of $x beginning at 2 ending at 7
>  (6)   $x[:]    // equivalent to $x (unspecified bounds)
> 
> In the action function, wrt to right-hand-side of the sequential-or, I
> need to determine:
>   a)  whether a range specifier has been provided at all
>       (given by (bool) at_c<1>(access_spec))
>   b)  whether the range specifier includes an explicit bound
>       (given by (bool) *at_c<1>(access_spec))
> 
> The left-hand-side of the sequential-or is either an (optional)
> 'range_begin' index if a range specifier is given or a lone 'index'
> specifier if no range specifier is given.
> 
> If the two optionals on the right-hand-side were collapsed would I not
> lose the ability to distinguish between (2) and (3)?

Yes, you're right, doh! Sorry for the noise.

Regards Hartmut

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