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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 76 |
1 files changed, 76 insertions, 0 deletions
@@ -30050,6 +30050,82 @@ In other words, the following equivalence holds: [f] -> nil .cble +.coNP Macros @ obtain* and @ obtain*-block +.synb +.mets (obtain* << forms *) +.mets (obtain*-block < name << forms *) +.syne +.desc +The +.code obtain* +and +.code obtain*-block +macros implement a useful variation of +.code obtain +and +.codn obtain-block . + +The +.code obtain* +macro differs from +.code obtain +in exactly one regard: prior to returning the function, it invokes +it one time, with the argument value +.codn nil . + +Thus, the following equivalence holds + +.cblk + (obtain* forms ...) <--> (let ((f (obtain forms ...))) + (call f) + f) +.cble + +In other words, the suspended block is immediately resumed, so that it executes +either to completion (in which case its value is discarded), or to its first +.code yield +or +.code yield-from +call (in which case the yielded value is discarded). + +Note: the +.code obtain* +macro is useful in creating suspensions which accept data rather than +produce data. + +The +.code obtain*-block +macro combines +.code obtain* +and +.code block +in the same manner that +.code obtain-block +combines +.code obtain +and +.codn block . + +.TP* Example: + +.cblk + ;; Pass three values into suspended block, + ;; which get accumulated into list. + (let ((f (obtain*-block nil + (list (yield nil) (yield nil) (yield nil))))) + (call f 1) + (call f 2) + (call f 3)) -> (1 2 3) + + ;; Under obtain, extra call is required: + (let ((f (obtain-block nil + (list (yield nil) (yield nil) (yield nil))))) + (call f nil) ;; execute block to first yield + (call f 1) ;; resume first yield with 1 + (call f 2) + (call f 3)) -> (1 2 3) +.cble + .coNP Macro @ suspend .synb .mets (suspend < block-name < var-name << body-form *) |