task: conditional execution

3 files changed, 67 insertions, 0 deletions

diff --git a/tutorial/index.md b/tutorial/index.md
index 62577a5f..a147b9bc 100644
--- a/tutorial/index.md
+++ b/tutorial/index.md
@@ -421,3 +421,34 @@ answer. Start from the first, and bump down if you need a hint.
 * tutorial/task10-hint1.mu
 * tutorial/task10-hint2.mu
 * tutorial/task10-hint3.mu
+
+## Task 11: conditionally executing statements
+
+Here's a fragment of Mu code:
+
+```
+{
+  compare x, 0
+  break-if-<
+  x <- copy 0
+}
+```
+
+The combination of `compare` and `break` results in the variable `x` being
+assigned 0 _if and only if_ its value was less than 0 at the beginning. The
+`break` family of instructions is used to jump to the end of an enclosing `{}`
+block, skipping all intervening instructions.
+
+To prepare for this task, read the sections in the Mu reference on
+[`compare`](https://github.com/akkartik/mu/blob/main/mu.md#comparing-values)
+and [branches](https://github.com/akkartik/mu/blob/main/mu.md#branches).
+
+Now make the tests pass in `tutorial/task11.mu`. The goal is to implement our
+colloquial understanding of the &ldquo;difference&rdquo; between two numbers.
+In lay English, we say the difference between the first-place and third-place
+runner in a race is two places. This answer doesn't depend on the order in
+which we mention the runners; the difference between third and first is also
+two.
+
+The section on [integer arithmetic](https://github.com/akkartik/mu/blob/main/mu.md#integer-arithmetic)
+is again worth referring to when working on this task.
diff --git a/tutorial/task11-solution.mu b/tutorial/task11-solution.mu
new file mode 100644
index 00000000..85140470
--- /dev/null
+++ b/tutorial/task11-solution.mu
@@ -0,0 +1,22 @@
+fn difference a: int, b: int -> _/eax: int {
+  var result/eax: int <- copy a
+  result <- subtract b
+  compare result, 0
+  {
+    break-if->=
+    result <- negate
+  }
+  return result
+}
+
+fn test-difference {
+  var result/eax: int <- difference 5, 3
+  check-ints-equal result, 2, "F - difference works"
+  result <- difference 3, 5
+  check-ints-equal result, 2, "F - difference is always positive"
+  result <- difference 6, 6
+  check-ints-equal result, 0, "F - difference can be 0"
+}
+
+fn main screen: (addr screen) {
+}
diff --git a/tutorial/task11.mu b/tutorial/task11.mu
new file mode 100644
index 00000000..0b9796e7
--- /dev/null
+++ b/tutorial/task11.mu
@@ -0,0 +1,14 @@
+fn difference a: int, b: int -> _/eax: int {
+}
+
+fn test-difference {
+  var result/eax: int <- difference 5, 3
+  check-ints-equal result, 2, "F - difference works"
+  result <- difference 3, 5
+  check-ints-equal result, 2, "F - difference is always positive"
+  result <- difference 6, 6
+  check-ints-equal result, 0, "F - difference can be 0"
+}
+
+fn main screen: (addr screen) {
+}