Delegates: Difference between revisions
→{{header|TXR}}: New section.
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}</syntaxhighlight>
=={{header|TXR}}==
<syntaxhighlight lang="txrlisp">;; TXR Lisp's :delegate implementation is hard delegation: the indicated
;; delegate object must exist and take the method call. To do soft
;; delegation, we develop a macro (delegate-or-fallback x y z)
;; which chooses x if x is an object which supports a z method,
;; or else chooses y.
(defun delegate-or-fallback-impl (del-inst fb-inst required-meth)
(let (del-type)
(if (and (structp del-inst)
(set del-type (struct-type del-inst))
(static-slot-p del-type required-meth)
(functionp (static-slot del-type required-meth)))
del-inst
fb-inst)))
(defmacro delegate-or-fallback (delegate-expr fallback-obj : required-meth)
^(delegate-or-fallback-impl ,delegate-expr ,fallback-obj ',required-meth))
;; With the above, we can use the defstruct delegate clause syntax:
;;
;; (:delegate source-method (obj) target-obj target-method)
;;
;; which writes a delegate method called source-method, that delegates
;; to target-method on target-obj. We calculate target-obj using
;; our macro and ensure that the delegator itself imlpements target-method.
(defstruct delegator ()
delegate
(:delegate operation (me) (delegate-or-fallback me.delegate me thing) thing)
(:method thing (me)
"default implementation"))
(defstruct delegate ()
(:method thing (me)
"delegate implementation"))
;; Tests:
;; no delegate
(prinl (new delegator).(operation))
;; struct delegate, but not with thing method
(prinl (new delegator delegate (new time)).(operation))
;; delegate with thing method
(prinl (new delegator delegate (new delegate)).(operation))</syntaxhighlight>
{{out}}
<pre>"default implementation"
"default implementation"
"delegate implementation"</pre>
=={{header|Vorpal}}==
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