6 The unwind–protection mechanism

Unwind–protection allows operations to be performed synchronously with respect to the dynamic extent of a call to thunk; the mechanism is sophisticated because there are multiple, sophisticated ways to exit the dynamic extent of a function call. The typical application is to release asynchronous resources, like input/output ports and database connections.

To understand the unwind–protection mechanism we must understand the concepts “dynamic extent of a function call” and “dynamic environment” as defined by the Scheme standard.

The unwind–protection mechanism described here is used by MMCK Exceptional Conditions in: the implementation of compensations (see compensations); the implementation of the syntax try (see syntaxes try).

NOTE The unwind protection mechanism may misbehave in some cases, so do not trust it blindly (see unwind problems).

This document describes version 0.1.0-devel.1 of MMCK Exceptional Conditions.