Basic pattern matching with TypeScript template literals

match.ts

export type KeysOfParams<S extends string> =
  S extends `${infer Before} :${infer Name} ${infer After}`
    ? KeysOfParams<Before> | Name | KeysOfParams<After>
    : S extends `:${infer Name} ${infer After}`
    ? Name | KeysOfParams<After>
    : S extends `${infer Before} :${infer Name}`
    ? KeysOfParams<Before> | Name
    : S extends `:${infer Name}`
    ? Name
    : never;

export type Params<S extends string> = {
  readonly [K in KeysOfParams<S>]: string;
};

export function match<S extends string>(
  template: S,
  input: string
): Params<S> | null {
  const ts = template.split(" ");
  const is = input.split(" ");

  if (ts.length !== is.length) {
    return null;
  }

  const params: Record<string, string> = {};

  for (let i = 0; i < ts.length; i++) {
    if (ts[i][0] === ":") {
      params[ts[i].slice(1)] = is[i];
    } else if (ts[i] !== is[i]) {
      return null;
    }
  }

  return params as Params<S>;
}

export function matches<S extends string>(
  template: S
): (input: string) => Params<S> | null {
  return match.bind(null, template);
}

const friend = matches("a friend named :first :last");

console.log(friend("a friend named Jane Doe"));
// => { first: "Jane", last: "Doe" }

console.log(friend("a friend"));
// => null