Add value-returning match expressions

README.md

@@ -108,18 +108,18 @@ enum PaymentResult {
 }
 
 fun describe(result: PaymentResult): String {
-    var description = ""
-    match (result) {
-        PaymentResult::Accepted(id) -> { description = id }
-        PaymentResult::Rejected(reason) -> { description = reason }
-        PaymentResult::Pending -> { description = "pending" }
+    return match (result) {
+        PaymentResult::Accepted(id) -> id
+        PaymentResult::Rejected(reason) -> reason
+        PaymentResult::Pending -> "pending"
     }
-    return description
 }
 ```
 
 Enum matches must contain each variant exactly once. Variant payload arity is
-checked during Gotlin compilation.
+checked during Gotlin compilation. A match used as an expression also requires
+every arm to return the same type. Block-style statement matches remain
+available for side effects.
 
 ## Null safety