Literal::Array#narrow type narrowing (#233)

lib/literal/array.rb

@@ -327,6 +327,23 @@ def minmax(...)
 		__with__(@__value__.minmax(...))
 	end
 
+	def narrow(type)
+		return self if __type__ == type
+
+		unless Literal.subtype?(type, of: @__type__)
+			raise ArgumentError.new("Cannot narrow #{@__type__} to #{type}")
+		end
+
+		@__value__.each do |item|
+			Literal.check(actual: item, expected: type) do |c|
+				c.fill_receiver(receiver: self, method: "#narrow")
+			end
+		end
+
+		@__type__ = type
+		self
+	end
+
 	def one?(...)
 		@__value__.one?(...)
 	end

test/array.test.rb

@@ -442,3 +442,33 @@
 	expect(Literal::Array(Integer).new(1, 2, 3).sum) == 6
 	expect(Literal::Array(String).new("1", "2", "3").sum(&:to_i)) == 6
 end
+
+test "#narrow" do
+	array = Literal::Array(Numeric).new(1, 2, 3)
+
+	refute Literal::Array(Integer) === array
+	array.narrow(Integer)
+	assert Literal::Array(Integer) === array
+end
+
+test "#narrow with same type" do
+	array = Literal::Array(Numeric).new(1, 2, 3)
+
+	assert Literal::Array(Numeric) === array
+end
+
+test "#narrow with wrong value" do
+	array = Literal::Array(Numeric).new(1, 2, 3.456)
+
+	expect {
+		array.narrow(Integer)
+	}.to_raise(Literal::TypeError)
+end
+
+test "#narrow with wrong type" do
+	array = Literal::Array(Integer).new(1, 2, 3)
+
+	expect {
+		array.narrow(Numeric)
+	}.to_raise(ArgumentError)
+end