[Voi-92] Basic type casting via match statements

README.md

@@ -240,17 +240,20 @@ for item in iterable
 
 ### Match Statements
 
+Match statements are used for type narrowing
+
 ```rust
-let x = 3
-match x
-  1 => print "One"
-  2 => print "Two"
-  3 => print "Three"
-  _ =>
-    // Match statements are exhaustive, they must cover every possible
-    // case. When not every case is covered, a default handler must be
-    // provided.
-    write "A number"
+obj Animal
+obj Cat extends Animal
+obj Dog extends Animal
+
+let dog = Dog {}
+
+match(dog)
+  Dog: print "Woof"
+  Cat: print "Meow"
+  else:
+    print "Blurb"
 ```
 
 ## Closures

reference/control-flow.md

@@ -0,0 +1,46 @@
+# Control Flow
+
+## Match
+
+Used to narrow types. Can operate on object to narrow
+child objects or on unions to narrow union members
+
+Signature(s):
+```
+fn match<T extends Object, U>(val: T, body: MatchBlock) -> U
+fn match<T extends Object, U>(val: T, bind_identifier: Identifier, body: MatchBlock) -> U
+```
+
+Example:
+```void
+obj Optional
+
+obj None extends Optional
+
+obj Some extends Optional {
+  value: i32
+}
+
+fn divide(a: i32, b: i32) -> Optional
+  if b == 0
+    None {}
+  else:
+    Some { value: a / b }
+
+fn main(a: i32, b: i32) -> String
+  let x = a.divide(b)
+  match(x)
+    Some: "The value is ${x}"
+    None: "Error: divide by zero"
+    else: "Bleh"
+```
+
+The second signature of match is useful when the value being matched against
+is not already bound to an identifier (i.e. dot pipelines):
+```void
+fn main(a: i32, b: i32) -> String
+  a.divide(b)
+    .match(x) // Here, match binds the result of the previous expression to x
+      Some: "The value is ${x}"
+      None: "Error: divide by zero"
+```

reference/types/objects.md

@@ -216,6 +216,32 @@ obj Cat extends Animal {
 }
 ```
 
+## Object Type Narrowing
+
+```void
+obj Optional
+
+obj None extends Optional
+
+obj Some extends Optional {
+  value: i32
+}
+
+fn divide(a: i32, b: i32) -> Optional
+  if b == 0
+    None { }
+  else:
+    Some { value: a / b }
+
+fn main(a: i32, b: i32)
+  a.divide(b)
+    .match(x)
+      Some:
+        log "The value is ${x}"
+      None:
+        log "Error: divide by zero"
+```
+
 # Traits
 
 Traits are first class types that define the behavior of a nominal object.

src/__tests__/compiler.test.ts

@@ -20,13 +20,19 @@ describe("E2E Compiler Pipeline", () => {
     const test2 = getWasmFn("test2", instance);
     const test3 = getWasmFn("test3", instance);
     const test4 = getWasmFn("test4", instance);
+    const test5 = getWasmFn("test5", instance);
+    const test6 = getWasmFn("test6", instance);
     assert(test1, "Test1 exists");
     assert(test2, "Test2 exists");
     assert(test3, "Test3 exists");
     assert(test4, "Test4 exists");
+    assert(test5, "Test3 exists");
+    assert(test6, "Test4 exists");
     t.expect(test1(), "test 1 returns correct value").toEqual(13);
     t.expect(test2(), "test 2 returns correct value").toEqual(1);
     t.expect(test3(), "test 3 returns correct value").toEqual(2);
     t.expect(test4(), "test 4 returns correct value").toEqual(52);
+    t.expect(test5(), "test 5 returns correct value").toEqual(21);
+    t.expect(test6(), "test 6 returns correct value").toEqual(-1);
   });
 });

src/__tests__/fixtures/e2e-file.ts

@@ -32,6 +32,12 @@ obj Pointy extends Vec {
   z: i32
 }
 
+obj Bitly extends Vec {
+  x: i32,
+  y: i32,
+  z: i32
+}
+
 fn get_x(vec: Vec)
   vec.x
 
@@ -44,6 +50,12 @@ fn get_member(vec: Point)
 fn get_member(vec: Pointy)
   vec.x
 
+fn get_num_from_vec_sub_obj(vec: Vec)
+  match(vec)
+    Pointy: get_member(vec)
+    Point: get_member(vec)
+    else: -1
+
 // Should return 13
 pub fn test1()
   let vec = Point { x: 1, y: 2, z: 13 }
@@ -59,10 +71,18 @@ pub fn test3()
   let vec = Vec { x: 1, y: 2 }
   vec.get_member()
 
-
 // Should return 52
 pub fn test4()
   let vec = Point { x: 52, y: 2, z: 21 }
   vec.get_x()
 
+// Test match type guard (Point case), should return 21
+pub fn test5()
+  let vec = Point { x: 52, y: 2, z: 21 }
+  get_num_from_vec_sub_obj(vec)
+
+// Test match type guard (else case), should return -1
+pub fn test6()
+  let vec = Bitly { x: 52, y: 2, z: 21 }
+  get_num_from_vec_sub_obj(vec)
 `;