adding filter to Tription

Author: bathalh

src/main/scala/spray/json/Tription.scala

@@ -25,12 +25,12 @@ sealed abstract class Tription[+T] extends Product
         else if( isNull ) Null
         else f( get )
 
+    /** return `Null` (not `Undefined`) if filter criteria don't match **/
+    final def filter(p: T => Boolean): Tription[T] =
+        if (!hasValue || p(this.get)) this else Null
+
     final def foreach[U](f: T => U): Unit =
         if( hasValue ) f( this.get )
-
-    // not sure whether to return Null or Undefined if the filter criteria are not met
-//    final def filter(p: T => Boolean): Tription[T] =
-//        if (!hasValue || p(this.get)) this else (Undefined/Null)
 }
 
 case class Value[+T](x: T) extends Tription[T] {

src/test/scala/spray/json/TriptionSpec.scala

@@ -76,6 +76,19 @@ class TriptionSpec extends Specification
             Null.flatMap( mapFunction ) mustEqual Null
             Value(value).flatMap( mapFunction ) mustEqual Value(value + append)
         }
+        "filter returns itself for Null and Undefined" in {
+            Undefined filter { _ => false } mustEqual Undefined
+            Null filter { _ => false } mustEqual Null
+            Undefined filter { _ => true } mustEqual Undefined
+            Null filter { _ => true } mustEqual Null
+        }
+        "filter returns Null if value does not match criteria" in {
+            Value(nextString) filter { _ => false } mustEqual Null
+        }
+        "filter returns itself if value matches criteria" in {
+            val value = nextString
+            Value(value) filter { _ => true } mustEqual Value(value)
+        }
         "foreach executes for value in a Value and does nothing otherwise" in {
             val sb = new StringBuilder
             def foreachFunction( x: Any ) = sb.append( x.toString )