9.2 Exceptions

See Exceptions for information on the Racket exception model. It is based on a proposal by Friedman, Haynes, and Dybvig [Friedman95].

Whenever a primitive error occurs in Racket, an exception is raised. The value that is passed to the current exception handler for a primitive error is always an instance of the exn structure type. Every exn structure value has a message field that is a string, the primitive error message. The default exception handler recognizes exception values with the exn? predicate and passes the error message to the current error display handler (see error-display-handler).

Primitive procedures that accept a procedure argument with a particular required arity (e.g., call-with-input-file, call/cc) check the argument’s arity immediately, raising exn:fail:contract if the arity is incorrect.

9.2.1 Raising Exceptions

If barrier? is true, then the call to the exception handler is protected by a continuation barrier, so that multiple returns/escapes are impossible. All exceptions raised by racket functions effectively use raise with a #t value for barrier?.

Breaks are disabled from the time the exception is raised until the exception handler obtains control, and the handler itself is parameterize-breaked to disable breaks initially; see Breaks for more information on breaks.

Examples:

> (with-handlers ([number? (lambda (n)                              (+ n 5))])     (raise 18 #t)) 23 > (define-struct (my-exception exn:fail:user) ()) > (with-handlers ([my-exception? (lambda (e)                                    #f)])     (+ 5 (raise (make-my-exception                   "failed"                   (current-continuation-marks))))) #f > (raise 'failed #t) uncaught exception: failed

• (error sym) creates a message string by concatenating "error: " with the string form of sym.

• (error msg v ...) creates a message string by concatenating msg with string versions of the vs (as produced by the current error value conversion handler; see error-value->string-handler). A space is inserted before each v.

• (error src frmat v ...) creates a message string equivalent to the string created by

  (format (string-append "~s: " frmat) src v ...)

In all cases, the constructed message string is passed to make-exn:fail, and the resulting exception is raised.

Examples:

> (error 'failed) error: failed > (error "failed" 23 'pizza (list 1 2 3)) failed 23 pizza (1 2 3) > (error 'method-a "failed because ~a" "no argument supplied") method-a: failed because no argument supplied

Examples:

> (raise-user-error 'failed) error: failed > (raise-user-error "failed" 23 'pizza (list 1 2 3)) failed 23 pizza (1 2 3) > (raise-user-error 'method-a "failed because ~a" "no argument supplied") method-a: failed because no argument supplied

In the first form, v is the value received by the procedure that does not have the expected type.

In the second form, the bad argument is indicated by an index bad-pos (counting from 0), and all of the original arguments v are provided (in order). The resulting error message names the bad argument and also lists the other arguments. If bad-pos is not less than the number of vs, the exn:fail:contract exception is raised.

Examples:

> (define (feed-cow animal)     (if (not (eq? animal 'cow))       (raise-type-error 'feed-cow "cow" animal)       "fed the cow")) > (feed-cow 'turkey) feed-cow: expected argument of type <cow>; given 'turkey > (define (feed-animals cow sheep goose cat)     (if (not (eq? goose 'goose))       (raise-type-error 'feed-animals "goose" 2 cow sheep goose cat)       "fed the animals")) > (feed-animals 'cow 'sheep 'dog 'cat) feed-animals: expects type <goose> as 3rd argument, given: 'dog; other arguments were: 'cow 'sheep 'cat

The arity-v value must be a possible result from procedure-arity, except that it does not have to be normalized (see procedure-arity? for the details of normalized arities); raise-arity-error will normalize the arity and use the normalized form in the error message. If name is a procedure, its actual arity is ignored.

The arg-v arguments are the actual supplied arguments, which are shown in the error message (using the error value conversion handler; see error-value->string-handler); also, the number of supplied arg-vs is explicitly mentioned in the message.

The name argument is usually #f when expr is provided; it is described in more detail below. The message is used as the main body of the error message.

The optional expr argument is the erroneous source syntax object or S-expression (but the expression #f cannot be represented by itself; it must be wrapped as a syntax object). The optional sub-expr argument is a syntax object or S-expression (again, #f cannot represent itself) within expr that more precisely locates the error. Both may appear in the generated error-message text if error-print-source-location is #t. Source location information in the error-message text is similarly extracted from sub-expr or expr when at least one is a syntax object and error-print-source-location is #t.

If sub-expr is provided and not #f, it is used (in syntax form) for the exprs field of the generated exception record, else the expr is used if provided and not #f. In either case, the syntax object is consed onto extra-sources to produce the exprs field, or extra-sources is used directly for exprs if neither expr nor sub-expr is provided and not #f.

The form name used in the generated error message is determined through a combination of the name, expr, and sub-expr arguments:

• When name is #f, and when expr is either an identifier or a syntax pair containing an identifier as its first element, then the form name from the error message is the identifier’s symbol.

• When name is #f and when expr is not an identifier or a syntax pair containing an identifier as its first element, then the form name in the error message is "?".

• symbol: When name is a symbol, then the symbol is used as the form name in the generated error message.

9.2.2 Handling Exceptions

Any procedure that takes one argument can be an exception handler. If the exception handler returns a value when invoked by raise, then raise propagates the value to the “previous” exception handler (still in the dynamic extent of the call to raise, and under the same barrier, if any). The previous exception handler is the exception handler associated with the rest of the continuation after the point where the called exception handler was associated with the continuation; if no previous handler is available, the uncaught-exception handler is used (see below). In all cases, a call to an exception handler is parameterize-breaked to disable breaks, and it is wrapped with call-with-exception-handler to install the exception handler that reports both the original and newly raised exceptions.

The default uncaught-exception handler prints an error message using the current error display handler (see error-display-handler) and then escapes by calling the current error escape handler (see error-escape-handler). The call to each handler is parameterized to set error-display-handler to the default error display handler, and it is parameterize-breaked to disable breaks. The call to the error escape handler is further parameterized to set error-escape-handler to the default error escape handler; if the error escape handler returns, then the default error escape handler is called.

When the current error display handler is the default handler, then the error-display call is parameterized to install an emergency error display handler that logs an error (see log-error) and never fails.

The new exception handler processes an exception only if one of the pred-expr procedures returns a true value when applied to the exception, otherwise the exception handler is invoked from the continuation of the with-handlers expression (by raising the exception again). If an exception is handled by one of the handler-expr procedures, the result of the entire with-handlers expression is the return value of the handler.

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

Before any predicate or handler procedure is invoked, the continuation of the entire with-handlers expression is restored, but also parameterize-breaked to disable breaks. Thus, breaks are disabled by default during the predicate and handler procedures (see Breaks), and the exception handler is the one from the continuation of the with-handlers expression.

The exn:fail? procedure is useful as a handler predicate to catch all error exceptions. Avoid using (lambda (x) #t) as a predicate, because the exn:break exception typically should not be caught (unless it will be re-raised to cooperatively break). Beware, also, of catching and discarding exceptions, because discarding an error message can make debugging unnecessarily difficult.

9.2.3 Configuring Default Handling

The error escape handler is normally called directly by an exception handler, in a parameterization that sets the error display handler and error escape handler to the default handlers, and it is normally parameterize-breaked to disable breaks. To escape from a run-time error in a different context, use raise or error.

Due to a continuation barrier around exception-handling calls, an error escape handler cannot invoke a full continuation that was created prior to the exception, but it can abort to a prompt (see call-with-continuation-prompt) or invoke an escape continuation (see call-with-escape-continuation).

The default error display handler displays its first argument to the current error port (determined by the current-error-port parameter) and extracts a stack trace (see continuation-mark-set->context) to display from the second argument if it is an exn value but not an exn:fail:user value.

The default error display handler in DrRacket also uses the second argument to highlight source locations.

To report a run-time error, use raise or procedures like error, instead of calling the error display handler directly.

The integer argument to the handler specifies the maximum number of characters that should be used to represent the value in the resulting string. The default error value conversion handler prints the value into a string (using the current global port print handler; see global-port-print-handler). If the printed form is too long, the printed form is truncated and the last three characters of the return string are set to “...”.

If the string returned by an error value conversion handler is longer than requested, the string is destructively “truncated” by setting the first extra position in the string to the null character. If a non-string is returned, then the string "..." is used. If a primitive error string needs to be generated before the handler has returned, the default error value conversion handler is used.

Calls to an error value conversion handler are parameterized to re-install the default error value conversion handler, and to enable printing of unreadable values (see print-unreadable).

9.2.4 Built-in Exception Types

The property value must be a procedure that accepts a single value—the structure type instance from which to extract source locations—and returns a list of srclocs. Some error display handlers use only the first returned location.

• source — An arbitrary value identifying the source, often a path (see Paths).

• line — The line number (counts from 1) or #f (unknown).

• column — The column number (counts from 0) or #f (unknown).

• position — The starting position (counts from 1) or #f (unknown).

• span — The number of covered positions (counts from 0) or #f (unknown).