maybe: define "filter"

[davidchambers]

This makes Maybe compatible with R.filter and, if ramda/ramda#799 is merged, with R.reject.

@joneshf, would you mind reviewing this to make sure I faithfully translated the first of the laws you gave me?

[joneshf]

I think this is actually a free theorem by parametricity.

[joneshf]

Looks like a good implementation.

I'm not sure what your overall idea is here, but I'd like to think that you could implement filter once and for all if you know something is both a Monad and a Monoid:

var filter = function(p, m) {
  return m.chain(function(x) {
    return p(x) ? m.of(x) : m.empty();
  });
}

Then you could put this on the prototype of whatever.

...
Maybe.prototype.filter = function(p) {
  return filter(p, this);
}
...

So you don't end up having to write something similar for other types that are Monad and Monoid. If you're not planning on using the fantasyland stuff then forget about it.

[davidchambers]

I'd like to think that you could implement filter once and for all if you know something is both a Monad and a Monoid

Great idea! Once I merge #8 Maybe will implement the Monad specification. I'll then open a pull request to have it implement the Monoid specification. Once I merge that pull request I'll update this one to use your filter function. :)

[davidchambers]

I've updated this pull request. Having Nothing and Just share their definition of filter is wonderful!