Added asSetFilterNulls

tribune-core/src/main/kotlin/com/sksamuel/tribune/core/collections/sets.kt

@@ -17,5 +17,20 @@ fun <I, A, E> Parser<I, A, E>.asSet(): Parser<Collection<I>, Set<A>, E> {
    }
 }
 
+/**
+ * Lifts an existing [Parser] to support sets of the input types supported by
+ * the underlying parser. Any nulls produced by the underlying parser will be filtered out
+ * without erroring.
+ *
+ * In other words, given a parser from I to A?, returns a parser from Set<I> to Set<A>.
+ *
+ * @return a [Parser]  that produces sets
+ */
+fun <I, A, E> Parser<I, A?, E>.asSetFilterNulls(): Parser<Collection<I>, Set<A>, E> {
+   return Parser { input ->
+      input.map { this@asSetFilterNulls.parse(it) }.sequence().map { it.filterNotNull().toSet() }
+   }
+}
+
 fun <I, A, E> Parser.Companion.set(elementParser: Parser<I, A, E>): Parser<Collection<I>, Set<A>, E> =
    elementParser.asSet()

tribune-core/src/main/kotlin/com/sksamuel/tribune/core/nulls.kt

@@ -25,7 +25,7 @@ fun <I, A, E> Parser<I, A?, E>.withDefault(ifNull: () -> A): Parser<I, A, E> =
 fun <I, A, E> Parser<I, A, E>.allowNulls(): Parser<I?, A?, E> = nullable()
 
 /**
- * Maps an existing non-nullable [Parser] I => A to accept null inputs I? => A?
+ * Maps an existing [Parser] I => A to accept null inputs I? => A?
  */
 fun <I, A, E> Parser<I, A, E>.nullable(): Parser<I?, A?, E> =
    Parser { input -> if (input == null) Either.Right(null) else this@nullable.parse(input) }

tribune-core/src/test/kotlin/com/sksamuel/tribune/core/DoubleTest.kt

@@ -28,7 +28,7 @@ class DoubleTest : FunSpec() {
       }
 
       test("parser should support doubles with nullable pass through") {
-         val p = Parser<String>().double { "not a double" }.allowNulls()
+         val p = Parser<String>().double { "not a double" }.nullable()
          p.parse("123.45").getOrNull() shouldBe 123.45
          p.parse(null).getOrNull() shouldBe null
       }