added prefer-method

2 files changed, 38 insertions, 12 deletions

diff --git a/src/clojure/boot.clj b/src/clojure/boot.clj
index 7b515309..69cdaf21 100644
--- a/src/clojure/boot.clj
+++ b/src/clojure/boot.clj
@@ -927,18 +927,15 @@
   [multifn dispatch-val & fn-tail]
   `(. ~multifn addMethod ~dispatch-val (fn ~@fn-tail)))
 
-;  `(let [pvar# (var ~multifn)]
-;     (. pvar# (commuteRoot (fn [#^clojure.lang.MultiFn mf#] 
-;                             (. mf# (assoc ~dispatch-val (fn ~@fn-tail))))))))
-
 (defmacro remove-method
   "Removes the method of multimethod associated	with dispatch-value."
  [multifn dispatch-val]
  `(. ~multifn removeMethod ~dispatch-val))
 
-;  `(let [pvar# (var ~multifn)]
-;      (. pvar# (commuteRoot (fn [#^clojure.lang.MultiFn mf#] 
-;                              (. mf# (dissoc ~dispatch-val)))))))
+(defmacro prefer-method
+  "Causes the multimethod to prefer matches of dispatch-val-x over dispatch-val-y when there is a conflict"
+  [multifn dispatch-val-x dispatch-val-y]
+  `(. ~multifn preferMethod ~dispatch-val-x ~dispatch-val-y))
 
 ;;;;;;;;; var stuff      
 
diff --git a/src/jvm/clojure/lang/MultiFn.java b/src/jvm/clojure/lang/MultiFn.java
index e956b761..de398429 100644
--- a/src/jvm/clojure/lang/MultiFn.java
+++ b/src/jvm/clojure/lang/MultiFn.java
@@ -13,11 +13,13 @@
 package clojure.lang;
 
 import java.util.Map;
+import java.util.Collection;
 
 public class MultiFn extends AFn{
 final public IFn dispatchFn;
 final public Object defaultDispatchVal;
 IPersistentMap methodTable;
+IPersistentMap preferTable;
 IPersistentMap methodCache;
 Object cachedHierarchy;
 
@@ -31,6 +33,7 @@ public MultiFn(IFn dispatchFn, Object defaultDispatchVal) throws Exception{
 	this.defaultDispatchVal = defaultDispatchVal;
 	this.methodTable = PersistentHashMap.EMPTY;
 	this.methodCache = methodTable;
+	this.preferTable = PersistentHashMap.EMPTY;
 	cachedHierarchy = null;
 }
 
@@ -46,6 +49,31 @@ synchronized public MultiFn removeMethod(Object dispatchVal) throws Exception{
 	return this;
 }
 
+synchronized public MultiFn preferMethod(Object dispatchValX, Object dispatchValY){
+	if(prefers(dispatchValY, dispatchValX))
+		throw new IllegalStateException(
+				String.format("Preference conflict: %s is already preferred to %s", dispatchValY, dispatchValX));
+	preferTable = preferTable.assoc(dispatchValX, RT.conj((IPersistentCollection) RT.get(preferTable,
+	                                                                                     dispatchValX,
+	                                                                                     PersistentHashSet.EMPTY),
+	                                                      dispatchValY));
+	resetCache();
+	return this;
+}
+
+private boolean prefers(Object x, Object y){
+	IPersistentSet xprefs = (IPersistentSet) preferTable.valAt(x);
+	return xprefs != null && xprefs.contains(y);
+}
+
+private boolean isA(Object x, Object y) throws Exception{
+	return RT.booleanCast(isa.invoke(x, y));
+}
+
+private boolean dominates(Object x, Object y) throws Exception{
+	return prefers(x, y) || isA(x, y);
+}
+
 private IPersistentMap resetCache(){
 	methodCache = methodTable;
 	cachedHierarchy = hierarchy.get();
@@ -72,14 +100,15 @@ private IFn findAndCacheBestMethod(Object dispatchVal) throws Exception{
 	for(Object o : methodTable)
 		{
 		Map.Entry e = (Map.Entry) o;
-		if(RT.booleanCast(isa.invoke(dispatchVal, e.getKey())))
+		if(isA(dispatchVal, e.getKey()))
 			{
-			if(bestEntry == null || RT.booleanCast(isa.invoke(e.getKey(), bestEntry.getKey())))
+			if(bestEntry == null || dominates(e.getKey(), bestEntry.getKey()))
 				bestEntry = e;
-			if(!RT.booleanCast(isa.invoke(bestEntry.getKey(), e.getKey())))
+			if(!dominates(bestEntry.getKey(), e.getKey()))
 				throw new IllegalArgumentException(
-						String.format("Multiple methods match dispatch value: %s -> %s and %s",
-						              dispatchVal, e.getKey(), bestEntry.getKey()));
+						String.format(
+								"Multiple methods match dispatch value: %s -> %s and %s, and neither is preferred",
+								dispatchVal, e.getKey(), bestEntry.getKey()));
 			}
 		}
 	if(bestEntry == null)