Error handling and continuations

Error handling and continuations

Arc's simplest mechanism for non-linear control flow is catch and throw. This allows execution to jump back to the catch expression, for example to exit a loop early.

arc> (catch (each x '(2 4 6 9 10 12) (prn "Examining " x) (if (odd x) (throw x))))
Examining 2
Examining 4
Examining 6
Examining 9
9

Arc also provides access to MzScheme's exception mechanism, which provides a way to trap errors or raise error.

The ccc function is equivalent to Scheme's call-with-current-continuation (often abbreviated as call/cc). Continuations are extremely powerful; for details see Teach Yourself Scheme in Fixnum Days, Chapter 20 of "On Lisp", Call with Current Continuation Patterns, or Advanced Scheme Techniques. In MzScheme, continuations are relatively expensive if the call stack is deep since capturing a continuation copies the stack.

point name [body ...] Creates a 'throw' function called name and executes body. If name is executed within body, that value will be returned from point. Otherwise, point has no effect and the value from body will be returned.	>(+ 10 (point throw (+ 20 (throw 3) 30))) 13
catch body Executes body and catches any value passed to throw. This is the same as point with the throw function's name predefined.	>(+ 10 (catch (+ 20 (throw 3) 30))) 13
ccc procedure Packages up the current continuation into an 'escape procedure' and passes it to the procedure. Equivalent to Scheme's call/cc or call-with-current-continuation.	>(ccc (fn (ep) (ep "bailout value") 42)) "bailout value"
protect during-procedure after-procedure Uses Scheme's dynamic-wind to ensure that after-procedure is executed whenever during-procedure exits.	>(protect (fn () (/ 1 0)) (fn () (prn "after"))) after Error: /: division by zero
after during-body [after-body ...] Ensures that after-body executes whenever during-body exits.
details exception Returns the message associated with an exception.	>(on-err (fn (ex) (details ex)) (err "boo")) Error: boo
err string ... Raises an exception with the given text.	>(err "Failure" 42) Error: Failure 42
on-err err-proc proc Executes proc. Calls err-proc if an exception occurs in proc. The exception is passed to err-proc	>(on-err (fn (ex) (string "caught " (details ex))) (fn () (/ 1 0))) "caught /: division by zero"
errsafe expr Executes expr, returning nil if an error occurs.	>(errsafe (/ 1 2)) 1/2 >(errsafe (/ 1 0)) nil

Copyright 2008 Ken Shirriff.