condtional multi-unwrap and guard clauses

examples/programs/conditional_unwrap/main.bro

@@ -1,6 +1,9 @@
-# Milestone 5: conditional unwrapping `if opt |v| { ... }`.
-# Tests a present optional binds and unwraps, a none takes the else branch, and
-# that optional pointers (?*T) unwrap the same way.
+# Milestone 5: conditional optional unwrapping, guards, and multi-unwrap.
+
+observe :: func(counter @mut i32, value ?i32) ?i32 {
+	counter^ = counter^ + 1
+	return value
+}
 
 main :: func() i32 {
 	total i32 = 0
@@ -34,5 +37,55 @@ main :: func() i32 {
 		total = total + 1000
 	}
 
+	# guarded multi-unwrap exposes every capture to the guard and then-block
+	age ?i32 = 2
+	if a and age |value, years : value + years == 42| {
+		total = total
+	} else {
+		total = total + 100
+	}
+
+	# parenthesized chains and three-value unwraps are equivalent
+	bonus ?i32 = 0
+	if (a and age and bonus) |value, years, extra : value + years + extra == 42| {
+		total = total
+	} else {
+		total = total + 100
+	}
+
+	# false guards use the else branch, or simply fall through without one
+	if a |value : value == 0| {
+		total = total + 100
+	} else {
+		total = total
+	}
+	if a |value : value == 0| {
+		total = total + 100
+	}
+
+	# a failed unwrap prevents later expressions from being evaluated
+	calls i32 = 0
+	if b and observe(&calls, age) |missing, observed| {
+		total = total + missing + observed
+	}
+	if calls != 0 {
+		total = total + 100
+	}
+
+	# short-circuiting also applies after an earlier successful unwrap
+	if a and b and observe(&calls, age) |value, missing, observed| {
+		total = total + value + missing + observed
+	}
+	if calls != 0 {
+		total = total + 100
+	}
+
+	# optional pointers participate in multi-unwrap and guards
+	if a and p |value, q : value == 40 and q^ == 0| {
+		total = total
+	} else {
+		total = total + 100
+	}
+
 	return total                   # expect exit code 42
 }