Implement Align type-class

[tonymorris]

trait Align[F[_]] extends Functor[F] {
  def alignWith[A, B, C]: (These[A, B] => C) => F[A] => F[B]
}

Addresses commonly brought up requests:

• https://twitter.com/igstan/status/379909084931035137
• http://stackoverflow.com/questions/3015962/zipping-with-padding-in-haskell

Subsumes the Scala standard library (ZipAll).

[xuwei-k]

I came up we can implement Traverse#align, but it does not work if stream is infinite. what do you think?

xuwei-k/scalaz@scalaz:2f0e2ff...xuwei-k:5eee5dd

[tonymorris]

Yeah I am not big into calling length.

[xuwei-k]

There is Align and Unalign implementation in Haskell
http://hackage.haskell.org/packages/archive/these/0.3/doc/html/Data-Align.html

[tonymorris]

Yep, We can do a bit better than that too I think :)

[xuwei-k]

def unalign[A, B, C]: (These[A, B] => C) => F[A] => F[B] is correct? What is the type C? .

I think unalign should be def unalign[A, B]: F[These[A, B]] => (F[A], F[B]) if generalize unalignStream and unalignList

[tonymorris]

Yeah I misnamed it. I mean this:

def alignWith[A, B, C](f: A \&/ B => C): (F[A], F[B]) => F[C]

[xuwei-k]

def alignWith[A, B, C](f: A \&/ B => C): (F[A], F[B]) => F[C]

I got it :)