src/utilities.lisp, src/class-finalize-impl.lisp: Improve error reports.

[sod] / src / utilities.lisp

diff --git a/src/utilities.lisp b/src/utilities.lisp
index 49ac335..1d58fa3 100644 (file)
--- a/src/utilities.lisp
+++ b/src/utilities.lisp
@@ -446,11 +446,13 @@
   (:documentation
    "Reports an inconsistency in the arguments passed to `merge-lists'.")
   (:report (lambda (condition stream)
-            (format stream "Merge inconsistency: failed to decide among ~A"
+            (format stream "Merge inconsistency: failed to decide between ~
+                            ~{~#[~;~A~;~A and ~A~:;~
+                                 ~@{~A, ~#[~;and ~A~]~}~]~}"
                     (merge-error-candidates condition)))))
 
 (export 'merge-lists)
-(defun merge-lists (lists &key pick (test #'eql))
+(defun merge-lists (lists &key pick (test #'eql) (present #'identity))
   "Return a merge of the given LISTS.
 
    The resulting list contains the items of the given LISTS, with duplicates
@@ -458,7 +460,10 @@
    the input LISTS in the sense that if A precedes B in some input list then
    A will also precede B in the output list.  If the lists aren't consistent
    (e.g., some list contains A followed by B, and another contains B followed
-   by A) then an error of type `inconsistent-merge-error' is signalled.
+   by A) then an error of type `inconsistent-merge-error' is signalled.  The
+   offending items are filtered for presentation through the PRESENT function
+   before being attached to the condition, so as to produce a more useful
+   diagnostic message.
 
    Item equality is determined by TEST.
 
@@ -500,7 +505,7 @@
                              candidates))
           (winner (cond ((null leasts)
                          (error 'inconsistent-merge-error
-                                :candidates candidates))
+                                :candidates (mapcar present candidates)))
                         ((null (cdr leasts))
                          (car leasts))
                         (pick