3.4.1 Debugging the Most Recent Error

When an error is signalled, Guile remembers the execution context where the error occurred. By default, Guile then displays only the most immediate information about where and why the error occurred, for example:

     (make-string (* 4 (+ 3 #\s)) #\space)
     -|
     standard input:2:19: In procedure + in expression (+ 3 #\s):
     standard input:2:19: Wrong type argument: #\s
     ABORT: (wrong-type-arg)
     
     Type "(backtrace)" to get more information or "(debug)" to enter the debugger.

However, as the message above says, you can obtain much more information about the context of the error by typing (backtrace) or (debug).

(backtrace) displays the Scheme call stack at the point where the error occurred:

     (backtrace)
     -|
     Backtrace:
     In standard input:
        2: 0* [make-string ...
        2: 1*  [* 4 ...
        2: 2*   [+ 3 #\s]
     
     Type "(debug-enable 'backtrace)" if you would like a backtrace
     automatically if an error occurs in the future.

In a more complex scenario than this one, this can be extremely useful for understanding where and why the error occurred. For more on the format of the displayed backtrace, see the subsection below.

(debug) takes you into Guile's interactive debugger, which provides commands that allow you to

• display the Scheme call stack at the point where the error occurred (the backtrace command — see Display Backtrace)
• move up and down the call stack, to see in detail the expression being evaluated, or the procedure being applied, in each frame (the up, down, frame, position, info args and info frame commands — see Frame Selection and Frame Information)
• examine the values of variables and expressions in the context of each frame (the evaluate command — see Frame Evaluation).

Use of the interactive debugger, including these commands, is described in Interactive Debugger.

• Backtrace Format: How to interpret a backtrace.