While you may define this Ord instance by defining the compare method, you should probably not do this since Ord is supposed to follow some rules, in particular transitivity "a < b and b < c means a < c" is assumed but false in your case. Those rules are important for many functions to work correctly (sort, minimum, maximum and so on...).

You may simply define another function like winAgainst :

Rock `winAgainst` Scissors = True

Scissors `winAgainst` Paper = True

Paper `winAgainst` Rock = True
_ `winAgainst` _ = False

or maybe a battle function that returns a Win | Loss | Tie...

(NB : All functions can be used as infix by surrounding their name with ` (antiquotes))

Le jeu. 30 juil. 2015 à 15:34, Nikita Danilenko <nda@informatik.uni-kiel.de> a écrit :

Hi Derek,

you can find the desired functions and type classes using Hoogle [1].
Your attempt at an Ord instance is basically the right idea, but you
need to define the function "compare" only [2]. From your syntax it
looks like you are trying to define a function called "ord" that
additionally uses "compare" (in a way, it cannot be used). Removing
"ord" in all three cases fixes your problem, although the resulting
function "compare" is not total.

The documentation of the type classes usually contains a description of
what functions you need to define. The fact that the "Show" type class
instance can be obtained using a function called "show" is a coincidence.

Best regards,

Nikita

[1] https://www.haskell.org/hoogle/

[2] http://hackage.haskell.org/package/base-4.8.1.0/docs/Prelude.html#t:Ord