Handling Errors Deterministically

See it run

(guard (e ((error-object? e) (error-object-message e))) (error "bad input"))

"bad input"

Walk through the pattern

1. Raise a guest error. (error "bad input") creates a catchable error object with a stable message.
2. Match structured data. guard tests error-object? rather than parsing rendered text.
3. Return a recovery value. The selected clause extracts the message, so the whole form evaluates to "bad input".

How to reason about it

• raise-continuable returns the handler result to the raise site. A non-continuable raise cannot return normally.
• Inspect error objects with the documented predicates and accessors rather than parsing rendered diagnostic text.
• When no guard clause matches, the original error is reraised unchanged.

Check yourself

What happens if no guard clause matches the error object?

A common mistake

ResourceLimit is not catchable, and host crashes are never converted into friendly guest errors.

Keep going

Diagnostics explains phases and codes. The decision-record guide shows what one recorded run refuses and what it still leaves to the application.

Diagnostics · Decision records