On Wed, Nov 10, 2010 at 4:51 PM, Shaping <[email protected]> wrote:
> If I want general-to-particular reading order, must I define a parsing word
> of my own?
>
> -b/2a <: b negated a/2 *
Defining a <: operator with that exact syntax is not possible. For
one, in Factor, new symbol names can only be created as part of syntax
forms, and so must come after a syntax word of some sort. The new name
"-b/2a" in this case comes before the <: syntax word. The other
problem is that you need some way to indicate where the expression
whose value is being bound ends.
You could, however, define a form with explicit opening and closing
tokens, for example:
[set -b/2a := b negated a/2 * ]
The definition for "[set" would look something like this:
--
SYNTAX: [set
scan locals get parse-def ! scan a token and parse it as a locals binding
":=" expect ! scan a token and raise an error if it's not ":="
\ ] parse-until ! scan and parse objects until the "]" word is parsed
swap suffix! append! ;
! append first the expression body, then the local definition to
the parser accumulator
! as if it had appeared as "expression :> name"
:: foo ( x y -- z ) [set z := x y + ] z ;
--
-Joe
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