either js

either.js

// Left represents error or bad case
var Left = function(x) {
  this.__value = x;
};

Left.of = function(x) {
  return new Left(x);
};

Left.prototype.map = function(f) {
  return this;
};

// Right represents success or good case
var Right = function(x) {
  this.__value = x;
};

Right.of = function(x) {
  return new Right(x);
};

Right.prototype.map = function(f) {
  return Right.of(f(this.__value));
}




//  getAge :: Date -> User -> Either(String, Number)
var getAge = curry(function(now, user) {
  var birthdate = computeDate(user.birthdate, 'YYYY-MM-DD');
  if (!birthdate.isValid()) return Left.of('Birth date could not be parsed');
  return Right.of(now.diff(birthdate, 'years'));
});

getAge(moment(), {
  birthdate: '2005-12-12',
});
// Right(9)

getAge(moment(), {
  birthdate: '20010704',
});
// Left('Birth date could not be parsed')