Merge pull request #1079 from hylo-lang/optional

Add 'Optional' to the standard library

Library/Hylo/Core/Optional.hylo

@@ -0,0 +1,26 @@
+/// Either an instance of `T` or the absence thereof.
+///
+/// Use `Optional` when you need to represent a value that may or may not be present.
+public typealias Optional<T> = Union<T, None<T>>
+
+/// The absence of an instance of `T`.
+public type None<T> {
+
+  /// Creates a value denoting the absence of an instance of `T`.
+  public memberwise init
+
+}
+
+public conformance None: Regular {
+
+  // TODO: Remove when #1078 is implemented.
+  public fun copy() -> Self {
+    None()
+  }
+
+  // TODO: Remove when #1078 is implemented.
+  public fun infix==(_ other: Self) -> Bool {
+    true
+  }
+
+}

Tests/LibraryTests/TestCases/OptionalTests.hylo

@@ -0,0 +1,11 @@
+//- compileAndRun expecting: success
+
+public fun main() {
+  var x: Optional<Int> = 42
+  let y = if let i: Int = x { i.copy() } else { 0 }
+  precondition(y == 42)
+
+  &x = None()
+  let z = if let i: Int = x { i.copy() } else { 0 }
+  precondition(z == 0)
+}