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4.4.5 Error Handling

When code being evaluated from the REPL hits an error, Guile enters a new prompt, allowing you to inspect the context of the error.

scheme@(guile-user)> (map string-append '("a" "b") '("c" #\d))
ERROR: In procedure string-append:
ERROR: Wrong type (expecting string): #\d
Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [1]>

The new prompt runs inside the old one, in the dynamic context of the error. It is a recursive REPL, augmented with a reified representation of the stack, ready for debugging.

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

scheme@(guile-user) [1]> ,bt
           1 (map #<procedure string-append _> ("a" "b") ("c" #\d))
           0 (string-append "b" #\d)

In the above example, the backtrace doesn’t have much source information, as map and string-append are both primitives. But in the general case, the space on the left of the backtrace indicates the line and column in which a given procedure calls another.

You can exit a recursive REPL in the same way that you exit any REPL: via ‘(quit)’, ‘,quit’ (abbreviated ‘,q’), or C-d, among other options.