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TS Playground

printf.ts

// >> inter(["a", "b", "c"], "d")
// ["a", "d", "b", "d", "c"]
function inter<T>(xs: T[], s: T): T[] {
  return xs.flatMap((x, i) => i === xs.length - 1 ? [x] : [x, s]);
}


// >> extractTok("ab,cd,ef", ",")
// ["ab", ",", "cd", ",", "ef"]
function extractTok(tok: string, template: string): string[] {
  return inter(tok.split(template), template);
}


// >> lex("a %d b %s c")
// ["a ", "%d", " b ", "%s", " c"]
function lex(format: string): string[] {
  return [format]
    .flatMap(_ => extractTok(_, "%d"))
    .flatMap(_ => extractTok(_, "%s"));
}


// >> parse<"a %d b %s c">
// [number, string]
type parse<s extends string> =
  s extends `%d${infer s}` ? [d: number, ...parse<s>] :
  s extends `%s${infer s}` ? [s: string, ...parse<s>] :
  s extends `${infer _}${infer s}` ? parse<s> :
  s extends `` ? [] :
  never;


// >> printf("Hello %s (id = %d)", "kana", 123)
// "Hello kana (id = 123)"
function printf<const Format extends string>(format: Format, ...args: parse<Format>) {
  let result = "";
  const arg = args[Symbol.iterator]();

  for (const tok of lex(format)) {
    switch (tok) {
      case "%d":
      case "%s":
        result += arg.next().value;
        break;
      default:
        result += tok;
    }
  }

  return result;
}


console.clear();
console.log(inter(["a", "b", "c"], "d"));
console.log(extractTok("ab,cd,ef", ","));
console.log(lex("a %d b %s c"));
console.log(printf("Hello %s (id = %d)", "kana", 123));