5.21.4.1 Single Stepping through a Procedure's Code

A good way to explore in detail what a Scheme procedure does is to set a trap on it and then single step through what it does. To do this, make and install a <procedure-trap> with the debug-trap behaviour from (ice-9 debugging ice-9-debugger-extensions).

The following sample session illustrates this. It assumes that the file matrix.scm defines a procedure mkmatrix, which is the one we want to explore, and another procedure do-main which calls mkmatrix.

     $ /usr/bin/guile -q
     guile> (use-modules (ice-9 debugger)
                         (ice-9 debugging ice-9-debugger-extensions)
                         (ice-9 debugging traps))
     guile> (load "matrix.scm")
     guile> (install-trap (make <procedure-trap>
                            #:procedure mkmatrix
                            #:behaviour debug-trap))
     guile> (do-main 4)
     This is the Guile debugger -- for help, type `help'.
     There are 3 frames on the stack.
     
     Frame 2 at matrix.scm:8:3
             [mkmatrix]
     debug> next
     Frame 3 at matrix.scm:4:3
             (let ((x 1)) (quote this-is-a-matric))
     debug> info frame
     Stack frame: 3
     This frame is an evaluation.
     The expression being evaluated is:
     matrix.scm:4:3:
       (let ((x 1)) (quote this-is-a-matric))
     debug> next
     Frame 3 at matrix.scm:5:21
             (quote this-is-a-matric)
     debug> bt
     In unknown file:
        ?: 0* [primitive-eval (do-main 4)]
     In standard input:
        4: 1* [do-main 4]
     In matrix.scm:
        8: 2  [mkmatrix]
        ...
        5: 3  (quote this-is-a-matric)
     debug> quit
     this-is-a-matric
     guile>

Or you can use Guile's Emacs interface (GDS), by using the module (ice-9 gds-client) instead of (ice-9 debugger) and (ice-9 debugging ice-9-debugger-extensions), and changing debug-trap to gds-debug-trap. Then the stack and corresponding source locations are displayed in Emacs instead of on the Guile command line.