Re: XML file formats .... was: Re: gEDA-user: Some footprints I tried to create

[John Doty <jpd@xxxxxxxxxxxxx>]

On Mar 15, 2007, at 8:32 AM, Evan Lavelle wrote:

> > However, I ended up implementing a flat, lexer-only solution,
> > which of course is possible only with reverse polish notation.   
> > So, if
> > you need a *small*, truely extensible (trivial, even), and easy to
> > write parser, that doesn't require even more complexities like
> > bison/yacc (not all platforms have them), then you might want to
> > consider the RPN option as well.
>
> <pedantic_warning>
> A lexer just tokenises an input stream for you; it matches the  
> input against a set of regular expressions that you specify, and  
> lets you execute some code when it finds a match.
>
> You could, in principle, implement an RPN (postfix) calculator  
> using something like lex, by just pushing matched tokens onto a  
> stack and popping them off when you match an operator. IMHO,  
> though, this would be pretty pointless. You might just as well  
> match "a + b" or "+ a b" and make a note to carry out evaluation  
> one or two tokens after the '+', instead of immediately. This then  
> gives you an infix or a prefix calculator, rather than a postfix  
> one. There's nothing special about the RPN case - it's just  
> marginally easier to implement, at the expense of readability.
>
> In practice, it's trivial (and normal) to just pass the matched  
> tokens to a parser such as yacc, which lets you check the structure  
> of the input, recover from errors, distinguish unary from binary  
> operators, evaluate complex expressions with precedence and  
> associativity, and so on. It's very difficult to build any smarts  
> into a lexer-based solution, but easy when you've got a parser on top.
>
> Where RPN is useful is where you have (very) simple stack-based  
> hardware, hence the usage in calculators.
> </pedantic_warning>
>
> Evan

Using the standard Unix compiler development tools for a Forth is  
swatting fles with a cannon.

John Doty              Noqsi Aerospace, Ltd.
jpd@xxxxxxxxx




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