Rebol [ Title: "Closure" File: %closure.r Date: 11-May-2009/19:13:51+2:00 Author: "Ladislav Mecir" Purpose: { CLOSURE is suggested instead of FUNC when you need functions exhibiting async behaviour, e.g. for: - View - Async Ports - Higher Order Functions Closures differ from "normal" Rebol functions by using a fresh context every time they are called. Rule of thumb: if your function is returning a new function, block, or a local word, you will be safe if it is a closure. } ] closure: func [ [catch] spec [block!] {Help string (opt) followed by arg words (and opt type and string)} body [block!] {The body block of the closure} /local spc item result ] [ spc: make block! 1 + (2 * length? spec) insert/only spc [throw] result: make block! 5 + length? spec insert result reduce [:do :make :function! spc body] parse spec [ any [ set item any-word! ( insert tail result to word! :item insert tail spc to get-word! :item insert/only tail spc [any-type!] ) | skip ] ] throw-on-error [make function! spec result] ] comment [ ; Examples: f-maker: func [x][does [x]] f-ok: f-maker "OK" f-bug: f-maker "BUG" f-ok ; == "BUG" c-maker: closure [x][does [x]] c-ok: c-maker "OK" c-bug: c-maker "BUG" c-ok ; == "OK" block: copy [] f: closure [x] [ if x = 2 [f 1] insert tail block 'x ] f 2 print block ; 1 2 ; Tests: f: closure [x [any-type!]] [type? get/any 'x] f () ; == unset! f make error! "" ; == error! f first [:x] ; == get-word! f first [x:] ; == set-word! f: closure [do make function! spc body] [ print [do make function! spc body] ] f 0 1 2 3 4 ; 0 1 2 3 4 f: closure [x [any-type!]] [return get/any 'x] type? f make error! "" ; == error! ]