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Re: XML file formats .... was: Re: gEDA-user: Some footprints I triedto create
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@noqsi.com
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