guard

guard

This operator places a new handler on the error continuation. If an error is thrown, this handler will be called with an error argument and a retry continuation. If the handler returns a value, this becomes the value of the (guard ...) form. If the handler calls the retry continuation with a new value, the computation that threw the error is retried. If the handler throws an error itself, it goes to the next handler up on the error continuation.

(proc fun-handler (err kontinue)
   (if (= err 'resume)
      (kontinue 42)
      (if (= err 'default)
         69
         (fail 'uh-oh))))

; no error

(+ 1
    (guard
        fun-handler
        (+ 2 3)))

; 6

; default
    
(+ 1
    (guard 
        fun-handler
        (+ 2 (fail 'default))))

; 70

; continue

(+ 1
    (guard
        fun-handler
        (+ 2 (fail 'resume))))

; 45

; abort

(+ 1
    (guard
        fun-handler
        (+ 2 (fail 'crap))))

; (runtime-error uh-oh)

Vaquero programmers should beware of infinite loops when retrying errors.