try-catch-finally

8.12

try-catch-finally🔗ℹ

Alex Knauth

A macro for catching exceptions and running teardown operations.

syntax
(try body ...+ catch-clause ... maybe-finally-clause)

catch-clause = (catch pred-expr => handler-expr)
| (catch (pred-expr id) handler-body ...+)
| (catch (id) handler-body ...+)
| (catch _ handler-body ...+)

maybe-finally-clause =
| (finally post-body ...+)

After evaluating the pred-exprs and handler-exprs, try evaluates the bodys with the new exception handlers, and then evaluates the post-bodys even if execution exits the bodys through an exception or continuation.

Examples:

> (try
    (raise-syntax-error #f "a syntax error")
    (catch (exn:fail:syntax? e)
      (displayln "got a syntax error")))
got a syntax error
> (try
    (raise-syntax-error #f "a syntax error")
    (catch (exn:fail:syntax? e)
      (displayln "got a syntax error"))
    (catch (exn:fail? e)
      (displayln "fallback clause")))
got a syntax error
> (try
    (displayln "at")
    (finally (displayln "out")))
at
out
> (let/cc up
    (try
      (displayln "at before")
      (up (void))
      (displayln "at after")
      (finally (displayln "out"))))
at before
out

syntax
(catch pred-expr => handler-expr)
(catch (pred-expr id) handler-body ...+)
(catch (id) handler-body ...+)
(catch _ handler-body ...+)

syntax
(finally post-body ...+)