match statements

examples/programs/match/main.bro

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+Animal :: enum {
+	dog
+	cat
+	bird
+}
+
+# tagged union over an existing enum (variants are a subset of the enum's members)
+Data :: union(Animal) {
+	dog  i32
+	bird i32
+}
+
+# tagged union with a synthesized tag enum
+Shape :: union(enum) {
+	square i32 # side
+	circle i32 # radius
+}
+
+# Statement match with payload capture; every variant returns, so the function needs
+# no trailing return (the desugared if/else chain covers all paths).
+describe :: func(d Data) i32 {
+	match d {
+		.dog |age|: return age + 1
+		.bird |wingspan|: return wingspan + 2
+	}
+}
+
+# Value match: single-expression arms yield implicitly, a block arm yields explicitly.
+area :: func(s Shape) i32 {
+	result :: match s {
+		.square |side|: side * side
+		.circle |r|: {
+			# 3 ~ pi, integer arithmetic
+			yield 3 * r * r
+		}
+	}
+	return result
+}
+
+main :: func() i32 {
+	dog Data = Data{ dog = 9 }
+	bird Data = Data{ bird = 38 }
+	total i32 = describe(dog) + describe(bird) # 10 + 40 = 50
+
+	# enum statement match, exhaustive without an `else`
+	a Animal = .bird
+	rank i32 = 0
+	match a {
+		.dog: rank = 1
+		.cat: rank = 2
+		.bird: rank = 3
+	}
+
+	# enum value match
+	legs :: match a {
+		.dog: 4
+		.cat: 4
+		.bird: 2
+	}
+
+	# integer match with a mandatory `else`
+	bucket i32 = 0
+	match rank {
+		1: bucket = 100
+		3: bucket = 5
+		else: bucket = 99
+	}
+
+	sq Shape = Shape{ square = 4 }
+	ci Shape = Shape{ circle = 2 }
+	shapes i32 = area(sq) + area(ci) # 16 + 12 = 28
+
+	# 50 + 3 + 2 + 5 + 28 = 88
+	return total + rank + legs + bucket + shapes - 88
+}