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REBOL [ Title: "Tail func" Date: 4-Oct-2012/11:08:03+2:00 Author: "Ladislav Mecir" File: %tail-func.r Purpose: {define tail-recursive functions} ] use [do-body do-tail-call] [ ; a helper function evaluating tail-func body do-body: func [ [throw] body [block!] ] [ while [true] [catch/name [return do body] 'tail-call] ] ; a helper function executing the tail call do-tail-call: func [ func-context [block! none!] 'tail-context [word! none!] ] [ if func-context [ set/any func-context second bind? tail-context ] throw/name none 'tail-call ] tail-func: func [ { Define a recursive user function with the supplied SPEC and BODY. The function can use a special TAIL-CALL local function to perform a tail-recursive function call. } [catch] spec [block!] {Help string (opt) followed by arg words (and opt type and string)} body [block!] {The body block of the function} /local the-function tail-call context-word context-block ] [ ; define a new 'tail-call local variable tail-call: use [tail-call] ['tail-call] ; bind the given BODY to "know" the 'tail-call variable body: bind/copy body tail-call ; find a local word in SPEC context-word: find spec any-word! if context-word [context-word: to word! first context-word] ; define the TAIL-CALL function set tail-call throw-on-error [ func spec compose [do-tail-call (context-word) (context-word)] ] ; define the function the-function: throw-on-error [ func spec reduce [context-word body] ] ; get the function context context-word: first second :the-function ; adjust the function body ; replace the context word in the function body by DO-BODY change second :the-function 'do-body ; adjust the TAIL-CALL body ; make sure the 'do-tail-call word is bound correctly change second get tail-call 'do-tail-call if context-word [ ; make the first argument of DO-TAIL-CALL a function context block context-block: first bind? context-word ; take care of refinements repeat i length? context-block [ poke context-block i to word! pick context-block i ] bind context-block context-word change/only at second get tail-call 2 context-block ] :the-function ] ]
halt ;; to terminate script if DO'ne from webpage