From 10a557db78474e81ba7ad39701a3611c65db0a8a Mon Sep 17 00:00:00 2001
From: Matthias Benkard <code@mail.matthias.benkard.de>
Date: Thu, 21 Feb 2008 22:54:47 +0100
Subject: Add a strictness option to protocols.

darcs-hash:28dd5077fc8cd98403f72f76668046a5b79bef21
---
 example.lisp   | 10 ++++++++--
 protocols.lisp | 33 +++++++++++++++++++++++++++++----
 2 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/example.lisp b/example.lisp
index b1dd46d..a8e0690 100644
--- a/example.lisp
+++ b/example.lisp
@@ -7,10 +7,12 @@
 
 
 (define-protocol printable ()
-  ((print-object * stream)))
+  ((print-object * stream))
+  (:strictness t))
 
 (define-protocol serialisable ()
-  ((serialise * stream)))
+  ((serialise * stream))
+  (:strictness t))
 
 (define-protocol additive ()
   ((add * *)
@@ -58,3 +60,7 @@
 
 (print (conforms-to-p 'b 'printable))             ;=> T
 (print (really-conforms-to-p 'b 'printable))      ;=> T
+
+(print (subtypep 'b 'printable))                  ;=> T
+(print (subtypep 'b 'additive))                   ;=> T
+(print (subtypep 'b 'serialisable))               ;=> NIL
diff --git a/protocols.lisp b/protocols.lisp
index 685afd1..0bb1fa6 100644
--- a/protocols.lisp
+++ b/protocols.lisp
@@ -4,6 +4,15 @@
 (defvar *protocols* (make-hash-table))
 
 
+(defclass protocol (standard-class)
+  ((methods :initarg :methods
+            :reader protocol-methods)
+   (strictness :initarg :strictness
+               :reader protocol-strictness
+               :initform '(nil)
+               :documentation "Only tag classes with this protocol if they actually (rather than allegedly) conform to it.")))
+
+
 (defun find-protocol (name &optional (errorp t))
   (find-class name errorp))
 
@@ -57,9 +66,23 @@
   nil)
 
 
-(defclass protocol (standard-class)
-  ((methods :initarg :methods
-            :reader protocol-methods)))
+(defmethod initialize-instance :around ((protocol protocol)
+                                        &rest initargs
+                                        &key (strictness
+                                              '(nil)
+                                              strictness-supplied-p)
+                                             direct-superclasses
+                                        &allow-other-keys)
+  (declare (ignore strictness))
+  (let ((new-initargs (copy-list initargs)))
+    (unless strictness-supplied-p
+      (setf (getf new-initargs :strictness)
+            (list (some #'protocol-strictp direct-superclasses))))
+    (apply #'call-next-method protocol new-initargs)))
+
+
+(defun protocol-strictp (protocol)
+  (car (protocol-strictness protocol)))
 
 
 (defmethod validate-superclass (class (superclass protocol))
@@ -150,7 +173,9 @@ methods ::= ((method-name [\\* | class-name]\\*)\\*)"
                        *actual-conformance*)
               really-conforming-p)
         (let ((original-class (find-class class-name)))
-          (unless (subtypep original-class protocol)
+          (unless (or (subtypep original-class protocol)
+                      (and (not really-conforming-p)
+                           (protocol-strictp protocol)))
             (handler-case
                 (let* ((new-class-name (gensym (symbol-name class-name)))
                        (temporary-class-name (gensym (symbol-name class-name)))
-- 
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