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tailfunc.r

REBOL [ Title: "Tail recursion" Date: 14-Nov-2001/15:58:27+1:00 Version: 1.0.0 File: %tailfunc.r Author: "Maarten Koopmans" Purpose: {Provides transparent tail recursive functions with refinement transferral. Source code 4 gurus only} Email: %m--koopmans2--chello--nl Web: http://www.vrijheid.net library: [ level: 'advanced platform: 'all type: [Tool function] domain: 'UI tested-under: none support: none license: none see-also: none ] ] tail-func: func [ {Returns a function that handles tail-recursion transparently.} args [block!] body [block!] /local meta-func meta-spec meta-body p1 p2 ] [ meta-spec: append/only copy [] args meta-body: append/only copy [] body ;matches refinements and copies refinements to our command p1: [ set r refinement! (either get bind to-word r 'comm [ append comm mold r ref-mode: on ] [ ref-mode: off ] ) ] ;matches words and copies their values to the statement if ref-mode = on p2: [ set w word! (if ref-mode [ append/only statement get bind to-word w 'local])] meta-func: copy [ ;The use context is accessible from the wrapper function that ;eliminates tail recursion. It plays the role of a stack frame ;ti implement a goto like behaviour in case of tail recursion use [ _*loop-detected _*myself _*innerfunc _*loops _*myspec _*myspec2 _*mycall] [ ;some static initialization of the use context varaiables _*loops: 0 _*loop-detected: false _*mycall: copy [] _*innerfunc: func (meta-spec) (meta-body) _*myspec: copy first :_*innerfunc _*myspec2: append copy _*myspec [/local ref-mode p1 p2 r w comm statement ret] insert/only _*myspec2 [catch] ;The function that is returned from the use context _*myself: func _*myspec2 [ ;How deep in a loop am I? _*loops: _*loops + 1 ;These parse rules extract how I am called ;(which refinements and so) p1: [(p1)] p2: [(p2)] ref-mode: on ;Ourt initial call comm: copy {_*innerfunc} ;Our initial statement statement: copy [] ;Generate our statement and call parse _*myspec [ any [ p1 | p2 ]] insert statement to-path comm ;Copy it in the use context so it survives ;a loop (_*mycall is the 'goto args) _*mycall: copy statement if _*loops = 2 [ _*loops: 1 _*loop-detected: true return ] ;Until we are no longer in loop-detection mode until [ _*loop-detected: false set/any 'ret do bind _*mycall '_*loops not _*loop-detected ] ;set/any 'ret pick ret 1 ;Use context cleanup _*loops: 0 _*loop-detected: false _*mycall: copy [] ;return our value return get/any 'ret ];_*myself: func ... ];use context ];meta-func ;return our function.... do compose/deep meta-func ] ;some samples rec-fun: tail-func [x [integer!]][x: x + 1 print x rec-fun x] rec-fun2: tail-func [x /y z] [ x: x + 1 either y [ print [ x z ] rec-fun2 x] [ print x rec-fun2/y x x ] ]
halt ;; to terminate script if DO'ne from webpage
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