Feature idea: Bi_traversable

[MattWindsor91]

Per Haskell.

This would have a similar setup to Bi_mappable inasmuch as having four arities (0, 1_left, 1_right, and 2, but would then follow Traversable for most of the structure; the generic basic input would be something like:

module On_monad (M : Monad.S) : sig
  val bi_map_m :
     ('l1, 'r1) t
  -> left:('l1 left -> 'l2 left M.t)
  -> right:('r1 right -> 'r2 right M.t)
  -> ('l2, 'r2) t M.t
end

We'd then have

• functors Fix_left/Fix_right/Fix_both for moving down rarities:
• functors Traverse_left/Traverse_right: Bi_traversable down to Traversable;
• the signature of Bi_mappable.Sn included in Bi_traversable.Sn, allowing use of the usual Bi_mappable functors.

[MattWindsor91]

Implemented now.