5.21.3.7 Tracing and (ice-9 debug)

The (ice-9 debug) module provides a tracing facility (see Tracing) that is roughly similar to that described here, but there are important differences.

• The (ice-9 debug) trace gives a nice pictorial view of changes in stack depth, by using indentation like this:

            [fact1 4]
            |  [fact1 3]
            |  |  [fact1 2]
            |  |  |  [fact1 1]
            |  |  |  |  [fact1 0]
            |  |  |  |  1
            |  |  |  1
            |  |  2
            |  6
            24
  

  However its output can only show the information seen here, which corresponds to (ice-9 debugging trace)'s trace/info procedure; it cannot be configured to show other pieces of information about the trap context in the way that the (ice-9 debugging trace) implementation can.

• The (ice-9 debug) trace only allows the tracing of procedure applications and their return values, whereas the (ice-9 debugging trace) implementation allows any kind of trap to be traced.

  It's interesting to note that (ice-9 debug)'s restriction here, which might initially appear to be just a straightforward consequence of its implementation, is also somewhat dictated by its pictorial display. The use of indentation in the output relies on hooking into the low level trap calls in such a way that the trapped application entries and exits exactly balance each other. The ice-9 debugging trace implementation allows traps to be installed such that entry and exit traps don't necessarily balance, which means that, in general, indentation diagrams like the one above don't work.

It isn't currently possible to use both (ice-9 debug) trace and (ice-9 debugging trace) in the same Guile session, because their settings of the low level trap options conflict with each other.