3.6 The context unwinding system

When the C code files in something, the doits in the file-in are evaluated in a different execution environment so that the file-in is protected from errors signalled in the doit; the same also happens, for example, when you do a C callin to Smalltalk.

Now, suppose that you want to evaluate something using Behavior's evaluate: method. Instead of using complex exception handling code, the unwinding system can be used to obtain that behavior with code as simple as this:

[ lastResult := evalFor perform: selector ] valueWithUnwind.

In fact, using valueWithUnwind arranges things so that an exception will resume execution after the block, instead of stopping it.

This system is quite low-level, so it should not be used in most cases: as a rule of thumb, use it only when the corresponding C code uses the prepareExecutionEnvironment, stopExecuting and finishExecutionEnvironment functions. The places where I'm using it obey this rule. They include the exception handling system, the ST parser, Behavior>>#evalString:to: and Behavior>>#evalString:to:ifError:.

The valueWithUnwind method is defined in `BlkClosure.st', but it mostly relies on code in the `ContextPart.st' file. Here are the methods involved in the unwinding mechanism: you can find descriptions in the Smalltalk source code. Note that all these methods are internal, this reference is provided just for sake of completeness.

• ContextPart class>>#unwind
• ContextPart class>>#unwind:
• ContextPart>>#mark
• ContextPart>>#returnTo:
• Process>>#unwindPoints