テンプレートリテラルによる解析

分割から構造化へ

type KeyValue<S> =
  S extends Tpl<infer K, '=', infer V> ? { key: K; value: V } : never;
// Tpl<K, '=', V> matches K, an equals sign, then V

type X = KeyValue<'name=alice'>; // { key: 'name'; value: 'alice' }

複数の推論箇所

type Request<S> =
  S extends Tpl<infer Method, ' ', infer Path>
    ? { method: Method; path: Path }
    : never;
// Tpl<Method, ' ', Path> matches the method, a space, then the path

type X = Request<'GET /users'>; // { method: 'GET'; path: '/users' }

推論した部分を絞り込む

type Method = 'GET' | 'POST';
type Parse<S> =
  S extends Tpl<(infer M extends Method), ' ', infer P>
    ? { method: M; path: P }
    : never;
// the inferred M is constrained to Method inside the template pattern

type X = Parse<'POST /x'>; // { method: 'POST'; path: '/x' }

キーと値のペアを解析する

type Pair<S> =
  S extends Tpl<infer K, '=', infer V> ? [K, V] : [S, ''];
// Tpl<K, '=', V> splits a 'key=value' string literal type

type X = Pair<'page=2'>; // ['page', '2']

ペアからオブジェクトを構築する

type FromPairs<T extends [string, string][]> =
  T extends [[infer K extends string, infer V], ...infer R extends [string, string][]]
    ? { [P in K]: V } & FromPairs<R>
    : {};

type X = FromPairs<[["a", "1"], ["b", "2"]]>; // { a: "1" } & { b: "2" }

クエリ文字列を解析する

type ParseQuery<S extends string> =
  FromPairs<{ [I in keyof SplitAmp<S>]: Pair<SplitAmp<S>[I] & string> }>;
// where SplitAmp splits on "&" (from previous lesson)

数値を取り出す

type IsNumeric<S extends string> =
  S extends Tpl<number> ? true : false;
// Tpl<number> is a template literal type that matches any numeric string

type X = IsNumeric<'42'>; // true
type Y = IsNumeric<'4a'>; // false

省略可能なセグメント

type ParsePath<S> =
  S extends Tpl<infer Base, '?', infer Query>
    ? { base: Base; query: Query }
    : { base: S; query: '' };
// Tpl<Base, '?', Query> matches an optional query after a question mark

type X = ParsePath<'/x?a=1'>; // { base: '/x'; query: 'a=1' }

空白とエッジケース

type CleanKey<S extends string> = Trim<S>;
// Apply Trim to each inferred segment before using it

これが強力な理由

type Event<S> =
  S extends Tpl<infer Domain, ':', infer Action>
    ? { domain: Domain; action: Action }
    : never;
// Tpl<Domain, ':', Action> matches 'domain:action'
type X = Event<'user:created'>; // { domain: 'user'; action: 'created' }

パーサーを組み合わせる

// Split -> Pair -> FromPairs is a parser pipeline at the type level

理解度チェック

まとめ