semigroup.js

/**
 * Semigroup
 * 
 * A Semigroup must have a concat method.
 * Just like JS Array and String
 * 
 * It simply takes 2 of the same datatype, concats them and returns that same datatype
 * 
 * The type signature for the concat method:
 * concat :: Semigroup a => a ~> a -> a
 * 
 * Spec: https://github.com/fantasyland/fantasy-land#semigroup
 */

const getDataType = (a) => {
  if (a === undefined) {
    return 'Undefined';
  }
  if (a === null) {
    return 'Null';
  }
  return (a).name || ((a).constructor && (a).constructor.name);
}

const typeGuard = (a, b) => {
  const aType = getDataType(a);
  const bType = getDataType(b);

  if (aType !== bType) {
    throw new TypeError(`Type mismatch. Expected ${aType} datatype`);
  }
}

const typeSafeConcat = (a, b) => {
  const type = getDataType(a);
  switch (type) {
    case 'Number':
    case 'BigInt':
    default:
      return a + b;
    case 'String':
    case 'Array':
      return a.concat(b);
    case 'Object':
      return { ...a, ...b };
    case 'Boolean':
      return Boolean(a + b);
    case 'Function':
      throw TypeError('Cannot concat datatype Function');
    case 'Symbol':
      throw TypeError('Cannot concat datatype Symbol');
    case 'Null':
      return null;
    case 'Undefined':
      return undefined;
  }
}

const Semigroup = (a) => ({
  a,
  toString: () => `Semigroup(${a})`,
  concat: (b) => {
    typeGuard(a, b);
    return Semigroup(
      typeSafeConcat(a, b),
    );
  },
});

Semigroup('Hello,').concat(' World!'); // => Semigroup(Hello, World!)
Semigroup(1).concat(3); // => Semigroup(4)
Semigroup(1).concat('3'); // TypeError
Semigroup(undefined).concat(undefined); // => Semigroup(undefined)
Semigroup(undefined).concat(null); // TypeError