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) >
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.
,bt) displays the Scheme call
stack at the point where the error occurred:
scheme@(guile-user) > ,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
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
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.