Vicare’s library (vicare) now exports a new keyword syntactic binding
super-protocol, as clause for define-record-type. All the changes
discussed here are in the master branch.
In a use of the syntax define-record-type, the definition clause:
(super-protocol ?expr)
allows the specification of a constructor protocol to be used by sub–types of a
record–type. Usually a sub–type make use of the protocol function built from the
constructor protocol specified by the clause protocol, but the clause
super-protocol overrides it.
The super-protocol works like protocol, its ?expr argument
must be an expression evaluating to a protocol function; the protocol function
accepts as single argument the constructor function of its super–type and it must
return the record instance.
Here is an example in which the record–type alpha is the super–type of
the record–type beta:
(define-record-type alpha
(fields a b)
(super-protocol
(lambda (make-record)
(lambda (a b)
(make-record (+ 10 a) (+ 20 b))))))
(define-record-type beta
(parent alpha)
(fields c d))
(let ((R (make-alpha 1 2)))
(values (alpha-a R)
(alpha-b R)))
⇒ 1 2
(let ((R (make-beta 1 2 3 4)))
(values (alpha-a R)
(alpha-b R)
(beta-c R)
(beta-d R)))
⇒ 11 22 3 4
the default constructor function make-alpha builds instances of
alpha with the default constructor; the constructor of beta
receives as constructor function the result of calling the super-protocol of
alpha.