Add [Either,Option].orNull. Closes #86.

lib/src/either.dart

@@ -9,6 +9,7 @@ abstract class Either<L, R> implements TraversableMonadOps<Either<L, dynamic>, R
 
   Either<L, R> orElse(Either<L, R> other()) => fold((_) => other(), (_) => this);
   R getOrElse(R dflt()) => fold((_) => dflt(), id);
+  R? get orNull => fold((_) => null, id);
   R operator |(R dflt) => getOrElse(() => dflt);
   Either<L2, R> leftMap<L2>(L2 f(L l)) => fold((L l) => left(f(l)), right);
   Option<R> toOption() => fold((_) => none(), some);

lib/src/option.dart

@@ -10,6 +10,7 @@ abstract class Option<A> implements TraversableMonadPlusOps<Option, A> {
   B cata<B, B2 extends B>(B ifNone(), B2 ifSome(A a)) => fold(ifNone, ifSome);
   Option<A> orElse(Option<A> other()) => fold(other, (_) => this);
   A getOrElse(A dflt()) => fold(dflt, (a) => a);
+  A? get orNull => fold(() => null, id);
   Either<B, A> toEither<B>(B ifNone()) => fold(() => left(ifNone()), (a) => right(a));
   Either<dynamic, A> operator %(ifNone) => toEither(() => ifNone);
   A operator |(A dflt) => getOrElse(() => dflt);

test/either_test.dart

@@ -71,6 +71,11 @@ void main() {
     expect(r.ensure((x) => x < 0, () => 0), right(0));
   });
 
+  test('getOrElseNull', () {
+    expect(left<String, int>('error').orNull, isNull);
+    expect(right<String, int>(42).orNull, 42);
+  });
+
   group("EitherM", () => checkMonadLaws(EitherM));
 
   //group("EitherTMonad+Id", () => checkMonadLaws(eitherTMonad(IdM)));

test/option_test.dart

@@ -41,6 +41,11 @@ void main() {
     expect(Option.unless(true, 42), none());
   });
 
+  test('orNull', () {
+    expect(none().orNull, isNull);
+    expect(some(42).orNull, 42);
+  });
+
   group("OptionM", () => checkMonadLaws(new OptionMonadPlus()));
 
   //group("OptionTMonad+Id", () => checkMonadLaws(optionTMonad(IdM)));