[ragel-users] scanning boolean expressions

[Wesley Moore ✆]

I was Was thinking about using Ragel to build a compiler for a programming language today and after looking at the clang sample linked on the website and the full C grammar I came to the conclusion that a scanner as the lever that fed tokens into a state machine with the grammar was the way I'd need to do it. In other words pretty much exactly what you described here. So I too would be interested to know if this is a sensible way to go about it.

Regards,
Wes

On 18/11/2010, at 5:51 PM, Brodie Thiesfield <brofield2 at jellycan.com> wrote:

> Hi,
> 
> Some advice please. I wanted to make a scanner/parser for a simple
> grammar. Simple boolean expressions with a variety of comparison
> operators.
> 
> I originally tried to combine the scanner with the parser, resulting
> in something like:
> 
> ============
> Tag := |*
>    '%' [a-z]+ { tag(ts,te); };
>    space;
>    *|;
> Op  := |*
>    "<" { op(LT); };
>    "<=" { op(LE); };
>    space;
>    *|;
> Val := |*
>    [0-9]+ { val(ts, te); };
>    space;
>    *|;
> Expr := Tag Op Val ;
> ============
> 
> It seemed exactly what I wanted. I wanted to express the valid order
> for things as in the state machine of Expr, but using the scanner to
> find the actual tokens, limiting them to only what is valid. However
> it results in the error "references to graph instantiations not
> allowed in expressions".
> 
> I eventually split it into two parts, a complete scanner and a parser
> based on token types. I take the results of the scanner (calls to
> tag(), op(), val(), etc) and pipe them into an instance of the state
> machine Expr, watching for state errors to see if the token wasn't
> valid.
> 
> ============
> Scanner := |*
>    # Tag
>    '%' [a-z]+ { tag(ts,te); };
>    # Op
>    "<" { op(LT); };
>    "<=" { op(LE); };
>    # Val
>    [0-9]+ { val(ts, te); };
>    space;
>    *|;
> 
> Tag = 'T';
> Op  = 'O';
> Val = 'V';
> Expr := Tag Op Val ;
> ============
> 
> I originally made it with a straight state machine, similar to the
> mongrel HTTP parser with lots of "mark" and field actions, but it
> tripped up on the difference between the operators "<" and "<=" for
> some reason giving me strange results there.
> 
> Although this works, it can't be the right way to do it. Would someone
> please give me pointers to a better way of doing it?
> 
> Regards,
> Brodie
> 
> _______________________________________________
> ragel-users mailing list
> ragel-users at complang.org
> http://www.complang.org/mailman/listinfo/ragel-users

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