Intro To FP Concepts, Part 3

script.js

// functor factory

const right = {
  of: val => functor(right, val),
  fmap: function(f){
    return right.of(f(this.val));
  }
}

const left = { 
  of: val => functor(left, val),
  fmap: function(f) { return this; } 
}

const maybe = { 
  of: val => functor(maybe, val),   // new !
  fmap: function(fn) {
    if(this.val === null) return maybe.of(null);
    return maybe.of(fn(this.val));  // changed
  }
}

const functor = (functorType, val) => {
  return Object.assign(
    Object.create(functorType), 
    { val: val }
  )
}

// fmap any functor !

const map = R.curry((fn, functor) => functor.fmap(fn));

// building blocks

const getWords = string => {
  var words = string.split(" ");
  if (words.length === 3) {
    return right.of(words);
  } else {
    return left.of("Not valid string")
  }
}

const getFirstName = words => words[1];
const getFirstLetter = string => string[0];

// composition

const firstInitial = map(
  R.pipe(getWords, map(getFirstName), map(getFirstLetter))
);

// try it - messy!
const user = maybe.of("Doc Emmett Brown");
console.log(firstInitial(user).val.val);   // "E"
const badUser = maybe.of("Doc Brown");
console.log(firstInitial(badUser).val.val);   // "Not valid string"
const noUser = maybe.of(null);
console.log(firstInitial(noUser).val);     // null 

scripts

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