September Morn Update

Task/Enforced-immutability/Swift/enforced-immutability-1.swift

@@ -0,0 +1,4 @@
+let a = 1
+a = 1 // error: a is immutable
+var b = 1
+b = 1

Task/Enforced-immutability/Swift/enforced-immutability-2.swift

@@ -0,0 +1,22 @@
+/// Value types are denoted by `struct`s
+struct Point {
+  var x: Int
+  var y: Int
+}
+
+let p = Point(x: 1, y: 1)
+p.x = 2 // error, because Point is a value type with an immutable variable
+
+/// Reference types are denoted by `class`s
+class ClassPoint {
+  var x: Int
+  var y: Int
+
+  init(x: Int, y: Int) {
+    self.x = x
+    self.y = y
+  }
+}
+
+let pClass = ClassPoint(x: 1, y: 1)
+pClass.x = 2 // Fine because reference types can be mutated, as long as you are not replacing the reference

Task/Enforced-immutability/Swift/enforced-immutability-3.swift

@@ -0,0 +1,9 @@
+// A common Swift beginner trap
+func addToArray(_ arr: [Int]) {
+  var arr = arr // Trying to modify arr directly is an error, parameters are immutable
+  arr.append(2)
+}
+
+let array = [1]
+addToArray(array)
+print(array) // [1], because value types are pass by copy, array is immutable