Make package export fantasy-fill

[phadej]

Related to #24 #56

With the fantasy fill you can fill the gaps in your pure functional, monadic fantasies.

var Id = require('./id.js');
var fantasyFill = require('./index.js');

function MyId(x) {
    this.value = x;
}

MyId.of = function (value) {
    return new MyId(value);
};

MyId.prototype.chain = function (action) {
    return action(this.value);
};

console.log('Id: ', fantasyFill(Id).join(", "));
console.log('MyId: ', fantasyFill(MyId).join(", "));

console.log('MyId 3 `map` \\x -> x * x: ', MyId.of(3).map(function (x) { return x * x; }));

Id:  Applicative, Apply, Chain, Functor, Monad, Monoid, Semigroup
MyId:  Applicative, Apply, Chain, Functor, Monad
MyId 3 `map` \x -> x * x:  { value: 9 }

[SimonRichardson]

This seems like it should go here if anything fantasy-sorcery

[phadej]

IMHO it would be nice to have some reasons to add fantasy-land as a dependency to the implementing libraries. So this package could be the spec, but also a common combinator library (like Control.Monad and Control.Applicative packages in Haskell's base).

Then, as a positive side-effect`, you'll see implementations as dependents on npmjs.org and they could use gemnasium to track spec updates. Of course fantasy-land could be added as dependency already now, but, as said, one more reason. This is related to the semver discussion.

[SimonRichardson]

@phadej now i've added the laws, would it be better that the laws are tested rather than just checking for a function?

var hasChain = typeof constructor.prototype.chain === 'function' && laws.chain.associativity(constructor)(equality)(1);

Then we know for certain that if it quacks like a duck and sounds like a duck then it is a duck.

[SimonRichardson]

It doesn't seem like this is getting any traction, if you want to keep it open, let me know.