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

From: joel de guzman (isis-tech_at_[hidden])
Date: 2001-05-29 22:19:57

From: "John Max Skaller" :

> > Spirit has virtually all the set operators
> 
> This may not be natural. Consider the grammar:
> 
> int = digit *
> float = digit + ["." digit *] 
> 
> where [] means 'optional'. Using 'set' operators, 
> I have to write
> 
> int = digit *
> float = digit+ ["." digit*] - int

Spirit resolves this with the Longest directive.
There's your maximal munch stuff.

> 
> whereas the 'natural' intepretation of the first form
> is to resolve ambiguities 'in order or writing'.
> The programming construction
> 
> if c1 ..
> else if c2 ..
> else if c3 ..
> 
> is actually equivalent to
> 
> if c1 ..
> if c2 - c1 ..
> if c3 - c2 - c2 ..
> 
> but the explicit sequencing of the first example
> simplifies the expression over the second, setwise,
> construction. This is just one example I can think of
> where set operators might not be clearest.
> 

Priorities can resolve this ambiguity.

Spirit has a default priority rule. Alternatives are short-circuited.
The right-most alternative has the highest priority. If this is not
desired, there's the Longest and Shortest directive.

Joel de Guzman