diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 55 |
1 files changed, 54 insertions, 1 deletions
@@ -27413,6 +27413,25 @@ and which create an abstraction which models the continuation as a suspended procedure supporting two-way communication of data. +Continuations raise the issue of what to do about unwinding. +The language Scheme provides the much criticized +.code dynamic-wind +operator which can execute initialization and clean-up code as +a continuation is entered and abandoned. \*(TX takes a simpler, +albeit risky approach. It provides a non-unwinding escape operator +.code sys:abscond-from +for use with continuations. Code which has captured a continuation +can use this operator to escape from the delimiting block without +triggering any unwinding among the frames between the capture point and the +delimiter. When the continuation is restarted, it will then do so +with all of the resources associated with it frames intact. +When the continuation executes normal returns within its context, +the unwinding takes place then. Thus tidy, "thread-like" use +of continuations is possible with a small measure of coding discipline. +Unfortunately, the absconding operator is dangerous: its use +breaks the language guarantee that clean-up associated with a form is done no +matter how a form terminates. + .TP* Notes: Delimited continuations resemble lexical closures in some ways. Both @@ -27548,6 +27567,37 @@ with named prompts. --> 24 .cble +.coNP Operator @ sys:abscond-from +.synb +.mets (sys:abscond-from < name <> [ value ]) +.syne +.desc +The +.code sys:abscond-from +operator closely resembles +.code return-from +and performs the same thing: it causes an enclosing block +.meta name +to terminate with +.meta value +which defaults to +.codn nil . + +However, unlike +.codn return-from , +.code sys:abscond-from +does not perform any unwinding. + +This operator should never be used for any purpose other than +implementing primitives for the use of delimited continuations. +It is used by the +.code yield-from +and +.code yield +operators to escape out of a block in which a continuation has +been captured. Neglecting to unwind is valid due to the expectation +that control will return into a restarted copy of that context. + .coNP Macros @ obtain and @ yield-from .synb .mets (obtain << forms *) @@ -27573,7 +27623,10 @@ are encapsulated in a special Finally, .code yield-from performs a non-local transfer to the same block, so that the yield object -appears as the result value of that block. +appears as the result value of that block. The non-local transfer is +performed abruptly, by the +.code sys:abscond-from +operator. An .code obtain |