Flowtype:如何创建类型保护函数?

type refinements 从函数。 如何创建 type guard

TypeScript示例:

function isString(arg: Showable): arg is string {
    return typeof arg === 'string';
}

流量

/* @flow */
type Showable = number | string;

// ok
function barOk (arg: Showable) {
  return typeof arg === 'string' ? arg.length : (arg + 1);
}

// type guard function
function isString(arg: Showable) {
    return typeof arg === 'string';
}

// Error
function barError (arg: Showable) {
  return isString(arg) ? arg.length : (arg + 1);
                         // ^ Cannot get `arg.length` because property `length` is missing in `Number`
}