src/class-*.lisp: Improve metaclass selection.

[sod] / src / class-utilities.lisp

diff --git a/src/class-utilities.lisp b/src/class-utilities.lisp
index 35c6d17..38cb75e 100644 (file)
--- a/src/class-utilities.lisp
+++ b/src/class-utilities.lisp
@@ -106,6 +106,51 @@
                            super sub)))))
 
 ;;;--------------------------------------------------------------------------
+;;; Metaclass inference.
+
+(export 'select-minimal-class-property)
+(defun select-minimal-class-property (supers key order default what
+                                     &key (present (lambda (x)
+                                                     (format nil "`~A'" x)))
+                                          allow-empty)
+  "Return the minimal partially-ordered key from the SUPERS.
+
+   KEY is a function of one argument which returns some interesting property
+   of a class.  The keys are assumed to be partially ordered by ORDER, a
+   function of two arguments which returns non-nil if its first argument
+   precedes its second.  If there is a unique minimal key then return it;
+   otherwise report a useful error and pick some candidate in an arbitrary
+   way; the DEFAULT may be chosen if no better choices are available.  If
+   ALLOW-EMPTY is non-nil, then no error is reported if there are no SUPERS,
+   and the DEFAULT choice is returned immediately.
+
+   In an error message, the keys are described as WHAT, which should be a
+   noun phrase; keys are filtered through PRESENT, a function of one
+   argument, before presentation.
+
+   The function returns two values: the chosen value, and a flag which is
+   non-nil if it was chosen without errors."
+
+  (let ((candidates (partial-order-minima (mapcar key supers) order)))
+    (cond ((and (null candidates) allow-empty)
+          (values default t))
+         ((and candidates (null (cdr candidates)))
+          (values (car candidates) t))
+         (t
+          (cerror* "No obvious choice for implicit ~A: ~
+                    ~{~#[root classes must specify explicitly~:;~
+                         candidates are ~
+                         ~#[~;~A~;~A and ~A~:;~@{~A, ~#[~;and ~A~]~}~]~]~:}"
+                   what (mapcar present candidates))
+          (dolist (candidate candidates)
+            (let ((super (find candidate supers :key key)))
+              (info-with-location super
+                                  "Direct superclass `~A' defined here ~
+                                   has ~A ~A"
+                                  super what (funcall present candidate))))
+          (values (if candidates (car candidates) default) nil)))))
+
+;;;--------------------------------------------------------------------------
 ;;; Miscellaneous useful functions.
 
 (export 'sod-subclass-p)