Material+UnknownCaseRepresentable.swift

enum Material: String {
    case wood, metal, glass, other
}

extension Material: Codable {}

extension Material: UnknownCaseRepresentable {
    static let unknownCase: Material = .other
}

Material(rawValue: "glass")
// glass

Material(rawValue: "stone")
// other

let json = """
["glass", "stone"]
"""
try? JSONDecoder().decode([Material].self, from: json.data(using: .utf8)!)
// [glass, other]