Question

I want to write an Ord instance for a data type Foo which delegates all comparison to a function bar :: Foo -> Bar where Bar is a data type that has an Ord instance available.

If I manually write this instance it looks like:

instance Ord Foo where
  compare x y
    | bar x == bar y = EQ
    | bar x <= bar y = LT
    | otherwise      = GT

Is there a more concise way to write this?

---

In Scala (with Scalaz), I can write:

 implicit val FooOrder: Order[Foo] = Order[Bar] contramap bar

Does Haskell have anything similar?

Answer 1

import Data.Ord

instance Ord Foo where
    compare = comparing bar

is the most concise version I can think of OTTOMH.

Slightly less concise, but better to generalise is

import Data.Function

instance Ord Foo where
    compare = compare `on` bar