Thanks very much for the help. I tried the program and it worked!
----- Original Message -----
From: <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Tuesday, February 08, 2000 3:30 PM
Subject: [REBOL] [Rebol_New] symbol manipulation Re:(3)
> Hello, jetroy!
>
> [EMAIL PROTECTED] wrote:
> >
> > How do you link up the BNF definition with the parse statement?
> >
>
> REBOL parse rules are conceptually similar to BNF, but you can (and
> must) embed REBOL code to take action on the parsed data -- unless,
> of course, you just want a pass/fail syntax check.
>
> >
> > I can't seem to find any simple examples. Thanks!
> >
>
> I'm not sure if this qualifies as "simple", but here's an example.
>
> =======================================================================
> REBOL []
>
> polydiff: function [
> expr [string!]
> ][
> alpha dig numb whsp gap coeff varbl expon result
> ][
> alpha: charset [#"a" - #"z"]
> dig: charset [#"0" - #"9"]
> numb: [some dig]
> whsp: charset " ^/^-"
> gap: [any whsp]
>
> either parse/all expr [
> gap [copy coeff numb | none (coeff: 1)]
> gap copy varbl alpha
> gap ["**" gap copy expon numb | none (expon: 1)]
> gap end
> ][
> coeff: to-integer coeff
> expon: to-integer expon
> coeff: coeff * expon
> expon: expon - 1
> result: copy ""
> either expon = 0 [
> append result coeff
> ][
> if coeff <> 1 [append result coeff]
> append result varbl
> if expon <> 1 [append result join "**" expon]
> ]
> result
> ][
> ""
> ]
> ]
> =======================================================================
>
> used as in
>
> >> polydiff "2x**3"
> == "6x**2"
> >> polydiff " 2 x ** 3 "
> == "6x**2"
> >> polydiff "x"
> == "1"
> >> polydiff "2x"
> == "2"
> >> polydiff "2x**2"
> == "4x
>
> Notes:
>
> * gap is there to gobble up optional whitespace
> * copy is there to save what matched into a word for later use
> * the alternatives with none set coefficient and exponent to default
> values if they are missing
> * the dozen or so lines for a successful parse are there to simplify
> the result expression by avoiding coefficient of 1, exponents of
> 0 or 1
>
> >
> > parse str [
> > factor (result: join result [ digit "*x**" digit - 1 ] ) ;this
> > doesn't work
> > ]
> >
>
> One reason why it doesn't work is that you're trying to use the word
> digit as if it were a variable containing the digit matched.
> It's not. It's just a test of whether a character is a digit.
>
> --
>
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