add Option.equal (#615)

@crusso 

I use this all the time when writing Matchers tests that require
equality checking. Would be nice to have in base 🙂

src/Option.mo

@@ -130,6 +130,13 @@ module {
     case _ { false }
   };
 
+  /// Returns true if the optional arguments are equal according to the equality function provided, otherwise returns false.
+  public func equal<A>(x : ?A, y : ?A, eq : (A, A) -> Bool) : Bool = switch (x, y) {
+    case (null, null) { true };
+    case (?x_, ?y_) { eq(x_, y_) };
+    case (_, _) { false }
+  };
+
   /// Asserts that the value is not `null`; fails otherwise.
   /// @deprecated Option.assertSome will be removed soon; use an assert expression instead
   public func assertSome(x : ?Any) = switch x {

test/Option.test.mo

@@ -1,4 +1,5 @@
 import Option "mo:base/Option";
+import Int "mo:base/Int";
 import Debug "mo:base/Debug";
 
 Debug.print("Option");
@@ -263,4 +264,15 @@ do {
       assert (false)
     }
   }
+};
+
+do {
+  Debug.print(" equal");
+
+  assert (Option.equal<Int>(null, null, Int.equal));
+  assert (Option.equal<Int>(?0, ?0, Int.equal));
+  assert (not Option.equal<Int>(?0, ?1, Int.equal));
+  assert (not Option.equal<Int>(?0, null, Int.equal));
+  assert (not Option.equal<Int>(null, ?0, Int.equal));
 }
+