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15.3 Signal Handling

The default superclass defines methods for managing error conditions. These methods all throw a signal for a particular error condition.

By implementing one of these methods for a class, you can change the behavior that occurs during one of these error cases, or even ignore the error by providing some behavior.

Function: slot-missing object slot-name operation &optional new-value ¶

Method invoked when an attempt to access a slot in object fails. slot-name is the name of the failed slot, operation is the type of access that was requested, and optional new-value is the value that was desired to be set.

This method is called from slot-value, set-slot-value, and other functions which directly reference slots in EIEIO objects.

The default method signals an error of type invalid-slot-name. See Signals.

You may override this behavior, but it is not expected to return in the current implementation.

This function takes arguments in a different order than in CLOS.

Function: slot-unbound object class slot-name fn ¶

Slot unbound is invoked during an attempt to reference an unbound slot. object is the instance of the object being reference. class is the class of object, and slot-name is the offending slot. This function throws the signal unbound-slot. You can overload this function and return the value to use in place of the unbound value. Argument fn is the function signaling this error. Use slot-boundp to determine if a slot is bound or not.

In clos, the argument list is (class object slot-name), but eieio can only dispatch on the first argument, so the first two are swapped.

Function: cl-no-applicable-method generic &rest args ¶

Called if there are no methods applicable for args in the generic function generic. args are the arguments that were passed to generic.

Implement this for a class to block this signal. The return value becomes the return value of the original method call.

Function: cl-no-primary-method generic &rest args ¶

Called if there are methods applicable for args in the generic function generic but they are all qualified. args are the arguments that were passed to generic.

Implement this for a class to block this signal. The return value becomes the return value of the original method call.

Function: cl-no-next-method generic method &rest args ¶

Called from cl-call-next-method when no additional methods are available. generic is the generic function being called on cl-call-next-method, method is the method where cl-call-next-method was called, and args are the arguments it is called by. This method signals cl-no-next-method by default. Override this method to not throw an error, and its return value becomes the return value of cl-call-next-method.