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5.21.3 Traps

The low level C code of Guile's evaluator can be configured to call out at key points to arbitrary user-specified procedures. These procedures, and the circumstances under which the evaluator calls them, are configured by the “evaluator trap options” interface (see Evaluator trap options), and by the trace and breakpoints fields of the “debug options” interface (see Debugger options). In principle this allows Scheme code to implement any model it chooses for examining the evaluation stack as program execution proceeds, and for suspending execution to be resumed later. Possible applications of this feature include breakpoints, runtime tracing, code coverage, and profiling.

Based on these low level trap calls, Guile provides a higher level, object-oriented interface for the manipulation of traps. Different kinds of trap are represented as GOOPS classes; for example, the <procedure-trap> class describes traps that are triggered by invocation of a specified procedure. A particular instance of a trap class — or trap object — describes the condition under which a single trap will be triggered, and what will happen then; for example, an instance of <procedure-trap> whose procedure and behaviour slots contain my-factorial and debug-trap would be a trap that enters the command line debugger when the my-factorial procedure is invoked.

The following subsections describe all this in detail, for both the user wanting to use traps, and the developer interested in understanding how the interface hangs together. A Quick Note on Terminology

It feels natural to use the word “trap” in some form for all levels of the structure just described, so we need to be clear on the terminology we use to describe each particular level. The terminology used in this subsection is as follows.

A good example of when it is important to be clear, is when we talk below of behaviours that should only happen once per low level trap. A single low level trap call will typically map onto the processing of several trap objects, so “once per low level trap” is significantly different from “once per trap”.