UIフレームワークを使わない実践例

UI以外のResult Builder

設定ビルダー

@resultBuilder
struct ConfigBuilder {
    static func buildBlock(_ parts: (String,String)...) -> [String:String] {
        var dict: [String:String] = [:]
        for (k,v) in parts { dict[k] = v }
        return dict
    }
}
func config(@ConfigBuilder _ body: () -> [String:String]) -> [String:String] { body() }

// Use it:
let cfg = config {
    ("ENV", "debug")
    ("API_URL", "https://api.example.com")
    ("RETRY", "3")
}
print(cfg["API_URL"] ?? "missing")

クエリビルダー

struct Where { let clauses: [String] }
@resultBuilder
struct WhereBuilder {
    static func buildBlock(_ parts: String...) -> Where { Where(clauses: parts) }
    static func buildArray(_ parts: [String]) -> Where { Where(clauses: parts) }
    static func buildEither(first: Where) -> Where { first }
    static func buildEither(second: Where) -> Where { second }
}
func whereClauses(@WhereBuilder _ body: () -> Where) -> Where { body() }

func select(_ table: String, where w: Where) -> String {
    let filter = w.clauses.isEmpty ? "" : " WHERE " + w.clauses.joined(separator: " AND ")
    return "SELECT * FROM \(table)\(filter);"
}

// Use:
let isProd = false
let q = select("users", where: whereClauses {
    "age > 18"
    if isProd { "status = 'active'" } else { "status <> 'deleted'" }
})
print(q)

テキスト/Markdownビルダー

@resultBuilder
struct TextBuilder {
    static func buildBlock(_ parts: String...) -> String { parts.joined(separator: "\n") }
    static func buildEither(first: String) -> String { first }
    static func buildEither(second: String) -> String { second }
    static func buildArray(_ parts: [String]) -> String { parts.joined(separator: "\n") }
}
func md(@TextBuilder _ body: () -> String) -> String { body() }

func h2(_ s: String) -> String { "## " + s }
func bullet(_ s: String) -> String { "- " + s }

let doc = md {
    h2("Release Notes")
    bullet("Faster build")
    bullet("Smaller binary")
}
print(doc)

検証ビルダー

struct ValidationResult { let ok: Bool; let messages: [String] }

typealias Rule = (String) -> String?   // nil = OK; otherwise error message

@resultBuilder
struct RulesBuilder {
    static func buildBlock(_ parts: Rule...) -> [Rule] { parts }
    static func buildArray(_ parts: [[Rule]]) -> [Rule] { parts.flatMap { $0 } }
}
func rules(@RulesBuilder _ body: () -> [Rule]) -> [Rule] { body() }

func validate(_ input: String, with rules: [Rule]) -> ValidationResult {
    var msgs: [String] = []
    for r in rules { if let m = r(input) { msgs.append(m) } }
    return ValidationResult(ok: msgs.isEmpty, messages: msgs)
}

// Use:
let rs = rules {
    { $0.isEmpty ? "Must not be empty" : nil }
    { $0.count < 3 ? "Too short" : nil }
}
let out = validate("hi", with: rs)
print(out.ok, out.messages)

設計のヒント

UI以外でのビルダーの使用例

まとめ