nth takes an optional not-found value, and destructuring uses nth with a nil not found value for sequential destructuring, nth without not-found value throws exception in all index-out-of-bounds cases including seqs

2 files changed, 58 insertions, 5 deletions

diff --git a/src/boot.clj b/src/boot.clj
index 589bb177..7c3ac083 100644
--- a/src/boot.clj
+++ b/src/boot.clj
@@ -758,9 +758,11 @@
 
 (defn nth
   "Returns the value at the index. get returns nil if index out of
-  bounds, nth throws an exception.  nth also works for strings, Java
-  arrays and Lists, and, in O(n) time, for sequences."
-  [coll index] (. clojure.lang.RT (nth coll index)))
+  bounds, nth throws an exception unless not-found is supplied.  nth
+  also works for strings, Java arrays, regex Matchers and Lists, and,
+  in O(n) time, for sequences."
+  ([coll index] (. clojure.lang.RT (nth coll index)))
+  ([coll index not-found] (. clojure.lang.RT (nth coll index not-found))))
 
 ;;map stuff
 
@@ -1992,7 +1994,7 @@ not-every? (comp not every?))
                                 (= firstb :as) (pb ret (second bs) gvec)
                                 :else (if seen-rest?
                                         (throw (new Exception "Unsupported binding form, only :as can follow & parameter"))
-                                        (recur (pb ret firstb  (list `nth gvec n))
+                                        (recur (pb ret firstb  (list `nth gvec n nil))
                                                (inc n)
                                                (rest bs)
                                                seen-rest?))))
diff --git a/src/jvm/clojure/lang/RT.java b/src/jvm/clojure/lang/RT.java
index 6784e4aa..10ab0016 100644
--- a/src/jvm/clojure/lang/RT.java
+++ b/src/jvm/clojure/lang/RT.java
@@ -597,7 +597,58 @@ static public Object nth(Object coll, int n){
 			if(i == n)
 				return seq.first();
 			}
-		return null;
+		throw new IndexOutOfBoundsException();
+		}
+	else
+		throw new UnsupportedOperationException("nth not supported on this type: " + coll.getClass().getSimpleName());
+}
+
+static public Object nth(Object coll, int n, Object notFound){
+	if(coll == null)
+		return notFound;
+	else if(coll instanceof IPersistentVector)
+		{
+		IPersistentVector v = (IPersistentVector) coll;
+		if(n < v.count())
+			return v.nth(n);
+		return notFound;
+		}
+	else if(coll instanceof String)
+		{
+		String s = (String) coll;
+		if(n < s.length())
+			return Character.valueOf(s.charAt(n));
+		return notFound;
+		}
+	else if(coll.getClass().isArray())
+		{
+		if(n < Array.getLength(coll))
+			return Array.get(coll, n);
+		return notFound;
+		}
+	else if(coll instanceof List)
+		{
+		List list = (List) coll;
+		if(n < list.size())
+			return list.get(n);
+		return notFound;
+		}
+	else if(coll instanceof Matcher)
+		{
+		Matcher m = (Matcher) coll;
+		if(n < m.groupCount())
+			return m.group(n);
+		return notFound;
+		}
+	else if(coll instanceof Sequential)
+		{
+		ISeq seq = ((IPersistentCollection) coll).seq();
+		for(int i = 0; i <= n && seq != null; ++i, seq = seq.rest())
+			{
+			if(i == n)
+				return seq.first();
+			}
+		return notFound;
 		}
 	else
 		throw new UnsupportedOperationException("nth not supported on this type: " + coll.getClass().getSimpleName());