[Javascript] Either Functor

Either Functor:

// API

Right(val) // resolve the value

Left(val) // return error message

Examples:

map(function(x) { return x + ; }, Right())

//=> Right(3)

map(function(x) { return x + ; }, Left(‘some message))

//=> Left(‘some message’)

var determineAge = function(user){

  return user.age ? Right(user.age) : Left("couldn’t get age");

}

var yearOlder = compose(map(add()), determineAge)

yearOlder({age: })

//=> Right(23)

yearOlder({age: null})

//=> Left("couldn’t get age")

var _ = R;

var P = PointFree;

var map = P.fmap;

var compose = P.compose;

var Maybe = P.Maybe;

var Identity = P.Id;

var Either = folktale.data.Either;

var Left = Either.Left;

var Right = Either.Right;

// Exercise 1

// ==========

// Write a function that uses checkActive() and showWelcome() to grant access or return the error

console.log("--------Start exercise 1--------")

var showWelcome = compose(_.add( "Welcome "), _.get('name'))

var checkActive = function(user) {

 return user.active ? Right(user) : Left('Your account is not active')

}

var ex1 = compose(map(showWelcome), checkActive);

assertDeepEqual(Left('Your account is not active'), ex1({active: false, name: 'Gary'}))

assertDeepEqual(Right('Welcome Theresa'), ex1({active: true, name: 'Theresa'}))

console.log("exercise 1...ok!")

// Exercise 2

// ==========

// Write a validation function that checks for a length > 3. It should return Right(x) if it is greater than 3 and Left("You need > 3") otherwise

console.log("--------Start exercise 2--------")

var ex2 = function(x) {

  if(x){

    return x.length >  ?

        Right(x) :

        Left("You need > 3");

  }else{

    return Left("You need > 3");

  }

}

assertDeepEqual(Right("fpguy99"), ex2("fpguy99"))

assertDeepEqual(Left("You need > 3"), ex2("..."))

console.log("exercise 2...ok!")

// Exercise 3

// ==========

// Use ex2 above and Either as a functor to save the user if they are valid

var save = function(x){ console.log("SAVED USER!"); return x; }

var ex3 = compose( map(save), ex2)

console.log("--------Start exercise 3--------")

assertDeepEqual(Right("fpguy99"), ex3("fpguy99"))

assertDeepEqual(Left("You need > 3"), ex3("duh"))

console.log("exercise 3...ok!")