From: Jim Pryor Date: Tue, 21 Dec 2010 15:31:23 +0000 (-0500) Subject: cps tweak, another Seasoned cite X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=commitdiff_plain;h=45bb412ca548c5cd02967d2b6c4d7ce2b0a88f0b cps tweak, another Seasoned cite Signed-off-by: Jim Pryor --- diff --git a/cps_and_continuation_operators.mdwn b/cps_and_continuation_operators.mdwn index f619fc0f..30dcc121 100644 --- a/cps_and_continuation_operators.mdwn +++ b/cps_and_continuation_operators.mdwn @@ -401,7 +401,9 @@ Here again is the CPS for `callcc`: [callcc (\k. body)] = \outk. (\k. [body] outk) (\v localk. outk v) -`callcc` is what's known as an *undelimited control operator*. That is, the continuations `outk` that get bound into our `k`s include all the code from the `call/cc ...` out to *and including* the end of the program. Calling such a continuation will never return any value to the call site. (See the technique employed in the `delta` example above, with the `(begin (let/cc k2 ...) ...)`, for a work-around.) +`callcc` is what's known as an *undelimited control operator*. That is, the continuations `outk` that get bound into our `k`s include all the code from the `call/cc ...` out to *and including* the end of the program. Calling such a continuation will never return any value to the call site. + +(See the technique employed in the `delta` example above, with the `(begin (let/cc k2 ...) ...)`, for a work-around. Also. if you've got a copy of *The Seasoned Schemer*, see the comparison of let/cc vs. "collector-using" (that is, partly CPS) functions at pp. 155-164.) Often times it's more useful to use a different pattern, where we instead capture only the code from the invocation of our control operator out to a certain boundary, not including the end of the program. These are called *delimited control operators*. A variety of these have been formulated. The most well-behaved from where we're coming from is the pair `reset` and `shift`. `reset` sets the boundary, and `shift` binds the continuation from the position where it's invoked out to that boundary.