Janet 1.10.1-9cfc3d9 Documentation
(Other Versions: 1.9.1 1.8.1 1.7.0 1.6.0 1.5.1 1.5.0 1.4.0 1.3.1 )

Error Handling

One of the main uses of fibers is to encapsulate and handle errors. Janet offers no direct try/catch mechanism like some languages, and instead builds error handling on top of fibers. When a function throws an error, Janet creates a signal and throws that signal in the current fiber. The signal will be propagated up the chain of active fibers until it hits a fiber that is ready to handle the error signal. This fiber will trap the signal and return the error from the last call to resume.

(defn block
  []
  (print "inside block...")
  (error "oops"))

# Pass the :e flag to trap errors (and only errors).
# All other signals will be propagated up the fiber stack.
(def f (fiber/new block :e))

# Get result of resuming the fiber in res.
# Because the fiber f traps only errors, we know
# that after resume returns, f either exited normally
# (has status :dead), or threw an error (has status :error)
(def res (resume f))

(if (= (fiber/status f) :error)
 (print "caught error: " res)
 (print "value returned: " res))

Janet also provides a simple macro try to make this a bit easier in the common case.

(try
 (do
   (print "inside block...")
   (error "oops")
   (print "will never get here"))
 ([err] (print "caught error: " err)))