add yield

examples/programs/yield/main.bro

@@ -0,0 +1,52 @@
+# Milestone 20: `yield` and value blocks.
+#
+# A `{ ... }` on the right of a declaration or assignment is a value block: its
+# final `yield <expr>` supplies the value (the block analogue of `return`). The
+# yielded value is captured before the block's defers run. Each section returns a
+# distinct code on failure; success falls through to 42.
+
+# Untyped `::`: the local's type is the yield's natural type.
+basic :: func() i32 {
+	x :: {
+		a :: 20
+		b :: 22
+		yield a + b
+	}
+	return x
+}
+
+# Typed `T =`: the yield coerces to the annotation.
+typed :: func() i64 {
+	x i64 = {
+		yield 100
+	}
+	return x
+}
+
+# The yielded value is captured before defers run: the defer mutates a block
+# local, but the captured value is unchanged.
+spill :: func() i32 {
+	v :: {
+		n i32 = 5
+		defer n = 999
+		yield n
+	}
+	return v # 5, not 999
+}
+
+# Reassignment into an existing mutable local.
+reassign :: func() i32 {
+	r i32 = 0
+	r = {
+		yield 7
+	}
+	return r
+}
+
+main :: func() i32 {
+	if (basic() != 42) return 101
+	if (typed() != 100) return 102
+	if (spill() != 5) return 103
+	if (reassign() != 7) return 104
+	return 42
+}