Scheme provides four procedures that take a condition type as input and
produce operations on the corresponding condition object. These are
reminiscent of the operations on record types that produce record
operators (see Records). Given a condition type it is possible to
generate: a constructor for instances of the type (using
condition-constructor); an accessor to extract the contents of a
field in instances of the type (using
predicate to test for instances of the type (using
condition-predicate); and a procedure to create and signal an
instance of the type (using
Notice that the creation of a condition object is distinct from signalling an occurrence of the condition. Condition objects are first-class; they may be created and never signalled, or they may be signalled more than once. Further notice that there are no procedures for modifying conditions; once created, a condition cannot be altered.
Returns a constructor procedure that takes as arguments values for the fields specified in field-names and creates a condition of type condition-type. Field-names must be a list of symbols that is a subset of the field-names in condition-type. The constructor procedure returned by
condition-constructorhas signature(lambda (continuation restarts . field-values) ...)
where the field-names correspond to the field-values. The constructor argument restarts is described in Restarts. Conditions created by the constructor procedure have
#ffor the values of all fields other than those specified by field-names.
For example, the following procedure
make-simple-warningconstructs a condition of type
condition-type:simple-warninggiven a continuation (where the condition occurred), a description of the restarts to be made available, a warning message, and a list of irritants that caused the warning:(define make-simple-warning (condition-constructor condition-type:simple-warning '(message irritants)))
Returns a procedure that takes as input a condition object of type condition-type and extracts the contents of the specified field-name.
error:bad-range-argumentif the field-name isn't one of the named fields of condition-type; the returned procedure will signal
error:wrong-type-argumentif passed an object other than a condition of type condition-type or one of its specializations.
If it is known in advance that a particular field of a condition will be accessed repeatedly it is worth constructing an accessor for the field using
condition-accessorrather than using the (possibly more convenient, but slower)
Returns a signalling procedure with parameters field-names. When the signalling procedure is called it creates and signals a condition of type condition-type. If the condition isn't handled (i.e. if no handler is invoked that causes an escape from the current continuation) the signalling procedure reduces to a call to default-handler with the condition as its argument.
There are several standard procedures that are conventionally used for default-handler. If condition-type is a specialization of
condition-type:error, default-handler should be the procedure
standard-error-handler. If condition-type is a specialization of
condition-type:warning, default-handler should be the procedure
standard-warning-handler. If condition-type is a specialization of
condition-type:breakpoint, default-handler should be the procedure