Exceptions

 

 

MzScheme supports the exception system proposed by Friedman, Haynes, and Dybvig. MzScheme's implementation extends that proposal by defining the specific exception values that are raised by each primitive error.

•   (raise exn) raises an exception, where exn represents the exception being raised. The exn argument can be anything; it is passed to the current exception handler. Any procedure that takes one argument can be an exception handler, but it is an error if the exception handler returns to its caller when invoked by raise. (If an exception handler returns, the current error display handler and current error escape handler are called directly to report the handler's mistake.)

  The default exception handler prints an error message using the current error display handler (see section 3.12) and then escapes by calling the current error escape handler (see section 3.13). If an exception is raised while the exeception handler is executing, an error message is printed using a primitive error printer and the primitive error escape handler is invoked.

•   (current-exception-handler) returns the current exception handler.
• (current-exception-handler f) installs the procedure f as the current exception handler.
• (with-handlers ((pred handler) ) expr ) is a syntactic form that evaluates the expr body, installing a new exception handler before evaluating the exprs and restoring the handler when a value is returned (or when control escapes from the expression). The new exception handler processes an exception only if one of the pred procedures returns a true value when applied to the exception, otherwise the original exception handler is invoked (by raising the exception again). If an exception is handled by one of the handler precedures, the result of the entire with-handlers expression is the return value of the handler.

  When an exception is raised during the evaluation of exprs, each predicate procedure pred is applied to the exception value; if a predicate returns a true value, the corresponding handler procedure is invoked with the exception as an argument. The predicates are tried in the order that they are specified. The pred and handler expressions are evaluated in the order that they are specified, before the first expr and before the exception handler is changed.

  Before any predicate or handler procedure is invoked, the continuation of the entire with-handlers expression is restored. The ``original'' exception handler (the one present before the with-handlers expression was evaluated) is therefore re-installed before any predicate or handler procedure is invoked.

This example defines a divide procedure that returns +inf.0 when dividing by zero instead of signalling an exception (other exceptions raised by / are signalled):

  (define div-w-inf
    (lambda (n d)
      (with-handlers ([exn:application:math:zero? 
                       (lambda (exn) +inf.0)])
        (/ n d))))

• Primitive Exceptions