Lecture 16 Summary, CSE 341 Spring 2004

Continuations

* A "continuation" represents the rest of computation.

* Some languages (Scheme, SML, Stackless Python, ...) provide
  "first-class continuations", giving you a binding for
  "forget your current computation and instead complete this
  computation with an argument 'filling the hole' "

* The key idiom for this is "escaping out of computations" for
  efficiency or exception handling.
  * Exception handlers can do these idioms too

* A related idiom does not need language support like Scheme's
  call/cc:  By passing a function for "what to do for the rest", we
  can do important things elegantly like iterate over trees.

You're not responsible for the following, but it's very cool:

* What makes let/cc more powerful than exceptions is: 
  * it's first-class
    (unlike exception handlers, you can make a list of continuations,
    etc.)
    (you can code up non-preemptive scheduling)
  * you can "time travel" -- re-enter a scope that was already exited
    -- there's an example in the code for class that iterates over
       trees this way