Merge pull request #4 from moonbitlang/0003-todo-cases

[ME-0003] Allow `...` in match cases

Author: bobzhang

proposals/0003-todo-cases.mbt.md

@@ -0,0 +1,43 @@
+# Support `...` in match cases
+
+* Proposal: [ME-0003](https://github.com/moonbitlang/moonbit-evolution/blob/0003-todo-cases/proposals/0003-todo-cases.mbt.md)
+* Author: [Yu-zh](https://github.com/Yu-zh)
+* Review and discussion: [Github issue](https://github.com/moonbitlang/moonbit-evolution/pull/4)
+
+## Introduction
+
+In MoonBit, we can use `...` as a placeholder for unimplemented code. For example:
+```moonbit
+fn init {
+  fn unimplemented() {
+    ...
+  }
+}
+```
+
+The compiler emits a warning when the `...` is used, but the code will be
+further compiled. If `...` is reached at runtime, the program will panic.
+
+## Allow `...` in match cases
+
+Currently, `...` is only allowed when a statement is expected. Therefore, if we
+want leave the code in match cases unimplemented, we have to write `_ => ...`:
+```moonbit
+pub fn f1(x: Int) -> Unit {
+  match x {
+    0 => ()
+    _ => ...
+  }
+}
+```
+
+We propose to extend the support to match cases so that we can write `...`
+directly:
+```moonbit
+pub fn f2(x: Int) -> Unit {
+  match x {
+    0 => ()
+    ...
+  }
+}
+```