add doc for loop label

next/language/fundamentals.md

@@ -779,6 +779,17 @@ A functional loop consumes arguments and returns a value. It is defined using th
 Currently in `loop exprs { ... }`, `exprs` is nonempty list, while `for { ... }` is accepted for infinite loop.
 ```
 
+### Labelled Continue/Break
+
+When a loop is labelled, it can be referenced from a `break` or `continue` from
+within a nested loop. For example:
+
+```{literalinclude} /sources/language/src/controls/top.mbt
+:language: moonbit
+:start-after: start loop label
+:end-before: end loop label
+```
+
 ## Iterator
 
 An iterator is an object that traverse through a sequence while providing access

next/sources/language/src/controls/top.mbt

@@ -211,7 +211,6 @@ test {
     i += j
   }
   assert_eq!(i, 45)
-
   let mut k = 0
   for l in 0..=10 {
     k += l
@@ -220,6 +219,40 @@ test {
 }
 // end for loop 10
 
+// start loop label
+test "break label" {
+  let mut count = 0
+  let xs = [1, 2, 3]
+  let ys = [4, 5, 6]
+  let res = outer~: for i in xs {
+    for j in ys {
+      count = count + i
+      break outer~ j
+    }
+  } else {
+    -1
+  }
+  assert_eq!(res, 4)
+  assert_eq!(count, 1)
+}
+
+test "continue label" {
+  let mut count = 0
+  let init = 10
+  let res =outer~: loop init {
+    0 => 42
+    i => {
+      for {
+        count = count + 1
+        continue outer~ i - 1
+      }
+    }
+  }
+  assert_eq!(res, 42)
+  assert_eq!(count, 10)
+}
+// end loop label
+
 // start guard 1
 fn guarded_get(array : Array[Int], index : Int) -> Int? {
   guard index >= 0 && index < array.length() else { None }
@@ -277,4 +310,4 @@ fn decide_sport(weather : String, humidity : Int) -> String {
 test {
   assert_eq!(decide_sport("sunny", 0), "tennis")
 }
-// end match 1
+// end match 1