Add information about labeled control flow (#79)

* Add information about labeled control flow

* Clean up some examples based on feedback and actually testing them

reference/src/control-flow/labeled-control-flow.md

@@ -1,9 +1,15 @@
 # Labeled Control Flow
 
-Move supports labeled control flow, allowing you to define and transfer control to specific labels
-in a function. For example, we can nest two loops and use `break` and `continue` with those labels
-to precisely specify control flow. You can prefix any `loop` or `while` form with a `'label:` form
-to allow breaking or continuing directly there.
+Move supports labeled control flow when writing both loops and blocks of code, allowing you
+to `break` and `continue` loops and `return` from blocks (which can be particularly helpful in the
+presence of macros).
+
+## Loops
+
+Loops allow you to define and transfer control to specific labels in a function. For example, we can
+nest two loops and use `break` and `continue` with those labels to precisely specify control flow.
+You can prefix any `loop` or `while` form with a `'label:` form to allow breaking or continuing
+directly there.
 
 To demonstrate this behavior, consider a function that takes nested vectors of numbers (i.e.,
 `vector<vector<u64>>`) to sum against some threshold, which behaves as follows:
@@ -19,18 +25,18 @@ outer label to escape both loops at once:
 fun sum_until_threshold(input: &vector<vector<u64>>, threshold: u64): u64 {
     let mut sum = 0;
     let mut i = 0;
-    let input_size = vector::length(vec);
+    let input_size = input.length();
 
     'outer: loop {
         // breaks to outer since it is the closest enclosing loop
         if (i >= input_size) break sum;
 
-        let vec = vector::borrow(input, i);
-        let size = vector::length(vec);
+        let vec = &input[i];
+        let size = vec.length();
         let mut j = 0;
 
         while (j < size) {
-            let v_entry = *vector::borrow(vec, j);
+            let v_entry = vec[j];
             if (sum + v_entry < threshold) {
                 sum = sum + v_entry;
             } else {
@@ -50,16 +56,93 @@ bodies of code. For example, if we were processing a large table where each entr
 that might see us continuing the inner or outer loop, we could express that code using labels:
 
 ```move
-'outer: loop {
+let x = 'outer: loop {
     ...
     'inner: while (cond) {
         ...
         if (cond0) { break 'outer value };
         ...
         if (cond1) { continue 'inner }
-        else if (cond2) { continue 'outer };
+        else if (cond2) { continue 'outer }
         ...
     }
-    ...
+        ...
+};
+```
+
+## Labeled Blocks
+
+Labeled blocks allow you to write Move programs that contain intra-function non-local control flow,
+including inside of macro lambdas and returning values:
+
+```move
+fun named_return(n: u64): vector<u8> {
+    let x = 'a: {
+        if (n % 2 == 0) {
+            return 'a b"even"
+        };
+        b"odd"
+    };
+    x
+}
+```
+
+In this simple example, the program checks if the input `n` is even. If it is, the program leaves
+the block labeled `'a:` with the value `b"even"`. If not, the code continues, ending the block
+labeled `'a:` with the value `b"odd"`. At the end, we set `x` to the value and then return it.
+
+This control flow feature works across macro bodies as well. For example, suppose we wanted to write
+a function to find the first even number in a vector, and that we have some macro `for_ref` that
+iterates the vector elements in a loop:
+
+```move
+macro fun for_ref<$T>($vs: &vector<$T>, $f: |&$T|) {
+    let vs = $vs;
+    let mut i = 0;
+    let end = vs.length();
+    while (i < end) {
+        $f(vs.borrow(i));
+        i = i + 1;
+    }
+}
+```
+
+Using `for_ref` and a label, we can write a lambda expression to pass `for_ref` that will escape the
+loop, returning the first even number it finds:
+
+```move
+fun find_first_even(vs: vector<u64>): Option<u64> {
+    'result: {
+        for_ref!(&vs, |n| if (*n % 2 == 0) { return 'result option::some(*n)});
+        option::none()
+    }
+}
+```
+
+This function will iterate `vs` until it finds an even number, and return that (or return
+`option::none()` if no even number exists). This makes named labels a powerful tool for interacting
+with control flow macros such as `for!`, allowing you to customize iteration behavior in those
+contexts.
+
+## Restrictions
+
+To clarify program behavior, you may only use `break` and `continue` with loop labels, while
+`return` will only work with block labels. To this end, the following programs produce errors:
+
+```
+fun bad_loop() {
+    'name: loop {
+        return 'name 5
+            // ^^^^^ Invalid usage of 'return' with a loop block label
+    }
+}
+
+fun bad_block() {
+    'name: {
+        continue 'name;
+              // ^^^^^ Invalid usage of 'break' with a loop block label
+        break 'name;
+           // ^^^^^ Invalid usage of 'break' with a loop block label
+    }
 }
 ```