messageCreationHelpers.ts

const messages = [
    system(`this is a system message
    
    It will end up typed as a literal because It is called as a function
    `),
    user`typed as
    
    a string This`,
    assistant(`
    will also 
    
    be typed
    as a literal`)
]










console.log(messages)
//            ^?


//----------------------------------
// implementation
//----------------------------------
export function system<T extends string>(
  literals: TemplateStringsArray | T,
  ...placeholders: unknown[]
) {
  return {
    role: "system" as const,
    content: dedent(literals, ...placeholders),
  };
}
export function user<T extends string>(
  literals: TemplateStringsArray | T,
  ...placeholders: unknown[]
) {
  return {
    role: "user" as const,
    content: dedent(literals, ...placeholders),
  };
}
export function assistant<T extends string>(
  literals: TemplateStringsArray | T,
  ...placeholders: unknown[]
) {
  return {
    role: "assistant" as const,
    content: dedent(literals, ...placeholders),
  };
}

export function dedent<T extends string>(
  templ: TemplateStringsArray | T,
  ...values: unknown[]
): typeof templ extends TemplateStringsArray ? string : T {
  let strings = Array.from(typeof templ === "string" ? [templ] : templ);

  // 1. Remove trailing whitespace.
  strings[strings.length - 1] = strings[strings.length - 1].replace(
    /\r?\n([\t ]*)$/,
    ""
  );

  // 2. Find all line breaks to determine the highest common indentation level.
  const indentLengths = strings.reduce<number[]>((arr, str) => {
    const matches = str.match(/\n([\t ]+|(?!\s).)/g);
    if (matches) {
      return arr.concat(
        matches.map((match) => match.match(/[\t ]/g)?.length ?? 0)
      );
    }
    return arr;
  }, []);

  // 3. Remove the common indentation from all strings.
  if (indentLengths.length) {
    const pattern = new RegExp(`\n[\t ]{${Math.min(...indentLengths)}}`, "g");

    strings = strings.map((str) => str.replace(pattern, "\n"));
  }

  // 4. Remove leading whitespace.
  strings[0] = strings[0].replace(/^\r?\n/, "");

  // 5. Perform interpolation.
  let string = strings[0];

  values.forEach((value, i) => {
    // 5.1 Read current indentation level
    const endentations = string.match(/(?:^|\n)( *)$/);
    const endentation = endentations ? endentations[1] : "";
    let indentedValue = value;
    // 5.2 Add indentation to values with multiline strings
    if (typeof value === "string" && value.includes("\n")) {
      indentedValue = String(value)
        .split("\n")
        .map((str, i) => {
          return i === 0 ? str : `${endentation}${str}`;
        })
        .join("\n");
    }

    string += indentedValue + strings[i + 1];
  });

  return string as any;
}

TS Playground Demo

const messages = [
     system`this is my system prompt
            it can span many lines
            and the extra padding will be stripped`,
     user`this is a user message
          it can do the same`,
    assistant(`All template strings can also be called as functions`)
];