h1. An Example

This comes from the "Advanced Ruby Course":http://pragmaticstudio.com/ruby/ 

h1. Continuation

<div style="width: 50%; float: right">
:code code/control/basic_continuation.rb[class=code-normal]
</div>

<div style="width: 50%">
  
* Construct using @callcc@
** pass it a block
** block receives @Continuation@ object
** invoke that object's @call@ method, and control passes to the end of that block

</div>


h1. Continuations Are Closures

:code code/control/closure_continuation.rb

h1. But...

:code code/control/closure_continuation_2.rb

h1. But...

<div>
:code code/control/closure_continuation_2.rb

<div style="position: relative; left: 60%; top: -3em; font-size: 400%; color: #6b6">Why?</div>
</div>



h1. Implementing throw...catch 

Let's try to implement throw and catch so that the following works as expected:

:code code/control/cc_throw_catch.rb[body]

h1. Implementing throw...catch

* We need a thread-local stack of all current catch blocks

* When we see a @catch@, we store the symbol and a continuation for the block on the stack. If that continuation gets called, execution resumes after the block (which is what @catch@ does)

* When we see a @throw@ we traverse it looking for the entry
with the corresponding system.

* When we find that entry, we call the corresponding continuation

&nbsp;

bq{font-size="50%"}. This implementation is based on code from Jim Weirich

h1. Catch and Throw

:code code/control/cc_throw_catch.rb[CC class=code-tiny]

h1. And the stack

:code code/control/cc_throw_catch.rb[stack class=code-tiny]

h1. Continuations.

* Can't invoke across threads

* Can carry stale state around with them

* Can use up a lot of memory

* Are not fun to debug

But...

* They'll impress your friends