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use-rule.r

Rebol [ Title: "Use-rule" File: %use-rule.r Author: ["Ladislav Mecir" "Gregg Irwin"] Date: 31-Oct-2010/9:48:59+1:00 Purpose: { Create a recursion and thread-safe parse rule with local variables. R2/R3 compatible. } Notes: { The USE-RULE function implements two USE variants as described in http://www.rebol.net/wiki/Parse_Project. The default, rebinding variant, rebinds the given rule every time it is used for matching (that is why we call it "rebinding variant"), while the /no-rebind variant binds the given rule just once. The difference between the rebinding and the /no-rebind variant is similar as the difference between closures and functions. The WORDS block is expected to contain only words. The implementation of the USE-RULE function does not use the latest parse enhancements to remain compatible with R2 as well as with R3. } ] use-rule: func [ { Create a recursion and thread-safe parse rule with local variables. R2/R3 compatible. } words [block!] {Local variables to use} rule [block!] {Parse rule} /no-rebind {Do not rebind the given RULE every time it is reused} ] [ ; we need some fresh local variables per every USE-RULE call make object! either no-rebind [[ ; We need the 'pos and 'success variables to transfer the parsing state ; between "inner" and "outer" PARSE calls. pos: none success: none ; The INNER-BODY function processes the given rule. inner-body: func [rule] [ ; If the parse of the given rule succeeds, ; we set the "outer parse position" ; to the point where the rule match ended, ; otherwise we use [end skip] for "parse failure" ; (for R2 compatibility). success: either parse pos [rule pos: to end] [ [:pos] ] [ [end skip] ] ] ; Define CONTEXT-FN's context to contain words specified by the caller. spec: copy [] unless empty? words [append spec to refinement! first words] append spec next words ; The CONTEXT-FN function calls the INNER-BODY function, ; supplying the given rule bound to the CONTEXT-FN function's context ; as the RULE argument to it. context-fn: func spec reduce [:inner-body rule] ; The result rule will be a "snapshot" of the block below, ; COPY/DEEP is needed to make sure the subsequent calls ; of the USE-RULE function do not modify the rule set 'rule copy/deep [pos: (context-fn) success] ]] [[ ; Define a new 'rebound-rule variable. ; (this block is processed by MAKE OBJECT! at the top) rebound-rule: none set 'rule reduce [ ; The new rule contains a paren! evaluation at the start. to paren! compose/only/deep [ ; We compose a copy of the given WORDS block ; as a WORDS argument into the USE call, ; and a deep copy of the given RULE into the USE BODY. ; Whenever the new rule is matched, the rule in the ; USE BODY is rebound by the USE call and deep copied. ; (to not be influenced by subsequent USE calls) rebound-rule: copy/deep use (copy words) [(copy/deep rule)] ] ; The rebound rule is used for matching by PARSE. 'rebound-rule ] ]] rule ] comment [ ; tests rule: [ "b" (print 1) | a: "a" rule "a" b: (print subtract index? b index? a) ] parse "aabaa" rule rule: use-rule [a b] [ "b" (print 1) | a: "a" rule "a" b: (print subtract index? b index? a) ] parse "aabaa" rule rule: use-rule/no-rebind [a b] [ "b" (print 1) | a: "a" rule "a" b: (print subtract index? b index? a) ] parse "aabaa" rule ]
halt ;; to terminate script if DO'ne from webpage