contrib: goops doc: calling next-method

[Marco Maggi]

Ciao,

  I propose the following to be appended to the
Next-method node in the GOOPS tutorial Texinfo.

@noindent
In the following example a class and a superclass define the
same
method:

@example
(use-modules (oop goops))

(define-class <a> ()
  one)

(define-class <b> (<a>)
  two)

(define-method (alpha (a <a>))
  "alpha a")

(define-method (alpha (b <b>))
  (list (next-method) "alpha b"))
@end example

@noindent
in the body of the @code{alpha<b>} function it is fine to invoke
@code{next-method} expecting it to invoke @code{alpha<a>}
because both
the @code{alpha} functions take the same number of
arguments. So the
result will be:

@example
(alpha (make <b>))
;; -> ("alpha a" "alpha b")
@end example

With the following definitions: the body of @code{alpha<b>}
invokes the
@code{alpha} function and an infinite loop will result:

@example
(define-method (alpha (b <b>))
  (list (alpha b) "alpha b"))
@end example

@noindent
the most specialised @code{alpha} function  is
@code{alpha<b>} itself.

With the following definitions:

@example
(use-modules (oop goops))

(define-class <a> ()
  one)

(define-class <b> (<a>)
  two)

(define-method (alpha (a <a>) c)
  (list "alpha a" c))

(define-method (alpha (b <b>))
  (list (next-method) "alpha b"))
@end example

@noindent
the code:

@example
(alpha (make <b>))
@end example

@noindent
will raise an error: the invocation of @code{alpha<b>} has
selected the
@code{alpha} functions that take only one parameter; the
invocation of
@code{next-method} in the body of @code{alpha<b>} will not
find any
other less specialised method; we will get an error even
with the
following:

@example
(define-method (alpha (b <b>))
  (list (next-method "string") "alpha b"))
@end example


It is possible to invoke @code{alpha<a>} by evaluating the
@code{alpha}
variable directly, with the appropriate number of parameters:

@example
(define-method (alpha (b <b>))
  (list (alpha b "string") "alpha b"))

(alpha (make <b>))
;; -> (("alpha a" "string") "alpha b")
@end example



--
Marco Maggi

"They say jump!, you say how high?"
Rage Against the Machine - "Bullet in the Head"