diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 39 |
1 files changed, 38 insertions, 1 deletions
@@ -37864,7 +37864,7 @@ 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: +.NP* Comparison with Lexical Closures Delimited continuations resemble lexical closures in some ways. Both constructs provide a way to return to some context whose evaluation @@ -37888,6 +37888,43 @@ environment. The continuation function returns to its caller when that entire restarted context terminates, whereas a closure returns to its caller as soon as the closure body terminates. +.NP* Differences in Compiled vs. Interpreted Behavior + +Delimited continuations in \*(TX expose a behavioral difference between +compiled and interpreted code. + +When a continuation is captured in compiled code, it captures not only the +bindings of lexical variables, but also potentially their current values +at the time of capture. Lexical variables captured in a continuation which are +changed after a continuation is captured may appear to revert to their +capture-time values inside the restored continuation. This is because in compiled +code, variables are allocated on the stack, which is copied as part of +creating a continuation. By contrast, interpreted code only maintains an +environment pointer on the stack; the lexical environment is a dynamically +allocated object whose contents aren't included in the continuation. +Code that doesn't mutate variables will not see a difference. +An additional complication is that when compiled code captures lexical +closures, captured variables are moved into dynamic storage. + +In continuation-based code which relies on mutation of lexical variables +created with +.code let +or +.codn let* , +the macros +.code hlet +and +.code hlet* +can be used instead. These macros create variable bindings whose storage is +outside of the stack. + +If the affected variables are other kinds of bindings such as +function parameters or variables created with specialized binding +constructs such as +.codn with-stream , +additional coding changes may be required to get interpreted code +working under compilation. + .coNP Function @ sys:capture-cont .synb .mets (sys:capture-cont < name < receive-fun <> [ context-form ]) |