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6.4.19 SRFI-35 - Conditions

SRFI-35 implements conditions, a data structure akin to records designed to convey information about exceptional conditions between parts of a program. It is normally used in conjunction with SRFI-34's raise: 

     (raise (condition (&message
                         (message "An error occurred"))))

Users can define condition types containing arbitrary information. Condition types may inherit from one another. This allows the part of the program that handles (or “catches”) conditions to get accurate information about the exceptional condition that arose.

SRFI-35 conditions are made available using: 

     (use-modules (srfi srfi-35))

The procedures available to manipulate condition types are the following:

— Scheme Procedure: make-condition-type id parent field-names

Return a new condition type named id, inheriting from parent, and with the fields whose names are listed in field-names. field-names must be a list of symbols and must not contain names already used by parent or one of its supertypes.

— Scheme Procedure: condition-type? obj

Return true if obj is a condition type.

Conditions can be created and accessed with the following procedures:

— Scheme Procedure: make-condition type . field+value

Return a new condition of type type with fields initialized as specified by field+value, a sequence of field names (symbols) and values as in the following example: 

          (let ((&ct (make-condition-type 'foo &condition '(a b c))))
            (make-condition &ct 'a 1 'b 2 'c 3))

Note that all fields of type and its supertypes must be specified.

— Scheme Procedure: make-compound-condition . conditions

Return a new compound condition composed of conditions. The returned condition has the type of each condition of conditions (per condition-has-type?).

— Scheme Procedure: condition-has-type? c type

Return true if condition c has type type.

— Scheme Procedure: condition-ref c field-name

Return the value of the field named field-name from condition c.

If c is a compound condition and several underlying condition types contain a field named field-name, then the value of the first such field is returned, using the order in which conditions were passed to make-compound-condition.

— Scheme Procedure: extract-condition c type

Return a condition of condition type type with the field values specified by c.

If c is a compound condition, extract the field values from the subcondition belonging to type that appeared first in the call to make-compound-condition that created the the condition.

Convenience macros are also available to create condition types and conditions.

— library syntax: define-condition-type type supertype predicate field-spec...

Define a new condition type named type that inherits from supertype. In addition, bind predicate to a type predicate that returns true when passed a condition of type type or any of its subtypes. field-spec must have the form (field accessor) where field is the name of field of type and accessor is the name of a procedure to access field field in conditions of type type.

The example below defines condition type &foo, inheriting from &condition with fields a, b and c: 

          (define-condition-type &foo &condition
            foo-condition?
            (a  foo-a)
            (b  foo-b)
            (c  foo-c))
— library syntax: condition type-field-bindings...

Return a new condition, or compound condition, initialized according to type-field-bindings. Each type-field-binding must have the form (type field-specs...), where type is the name of a variable bound to condition type; each field-spec must have the form (field-name value) where field-name is a symbol denoting the field being initialized to value. As for make-condition, all fields must be specified.

The following example returns a simple condition: 

          (condition (&message (message "An error occurred")))

The one below returns a compound condition: 

          (condition (&message (message "An error occurred"))
                     (&serious))

Finally, SRFI-35 defines a several standard condition types.

— Variable: &condition

This condition type is the root of all condition types. It has no fields.

— Variable: &message

A condition type that carries a message describing the nature of the condition to humans.

— Scheme Procedure: message-condition? c

Return true if c is of type &message or one of its subtypes.

— Scheme Procedure: condition-message c

Return the message associated with message condition c.

— Variable: &serious

This type describes conditions serious enough that they cannot safely be ignored. It has no fields.

— Scheme Procedure: serious-condition? c

Return true if c is of type &serious or one of its subtypes.

— Variable: &error

This condition describes errors, typically caused by something that has gone wrong in the interaction of the program with the external world or the user.

— Scheme Procedure: error? c

Return true if c is of type &error or one of its subtypes.