Consideration to add `ordering` function to `Data.Ord`.

Ordering.md

Good day,

In short, I think it would be nice to add the following ordering function to the Data.Ord module:

-- | Case analysis for the 'Ordering' type. @'ordering' x y z o @ evaluates to @x@
-- when @o@ is 'LT', @y@ when @o@ is EQ, and evaluates to @z@ when @o@ is 'GT'.
ordering :: a -> a -> a -> Ordering -> a
ordering lt _  _  LT = lt
ordering _  eq _  EQ = eq
ordering _  _  gt GT = gt

This would be essentially analogous to the bool function from Data.Bool:

bool :: a -> a -> Bool -> a
bool f _ False = f
bool _ t True  = t

Because the bool function is the case-analysis for the Bool data type, and the order of the parameters matches the order in which the constructors are defined:

Data Bool = False | True

I decided it would be consistent to follow the same approach for the ordering implementation and so I use the particular ordering (no pun intended) for the parameters to also match the order of the constructors:

Data Ordering = LT | EQ | GT

I will also add an example use case, here is how show could be implemented using the proposed function (of course, we momentarily assume it is not derived, for sake of simplicity):

instance Show Ordering where
  show :: Ordering -> String
  show = ordering "LT" "EQ" "GT"

Please let me know if you would wish for me to share more compelling examples, I have a few, but they are longer (so not here included for brevity's sake).

I hope the formatting came out okay, I will link to a gist[1] just in case.

If this sounds acceptable, please let me know how to proceed (would this fall under "Core Libraries Proposal"[2]? I don't necessarily see adding this function as a breaking change, so I'm not sure, but I can surely fill one out if it's appropriate); otherwise, thank you for your time and consideration.

Respectfully,

Ian Treyball

[1] https://gist.github.com/subttle/b49762a929f25e349381ef161bbc33d0

[2] https://github.com/haskell-core/core-libraries-proposals