In both cases, it's not actually creating a typeclass instance. However, because (<=) from Ord is declared in GHC.Classes, you're able to create a new (but completely unrelated) function named (<=). The fully qualified names for these would be GHC.Classes.<= and YourModule.<=, so they don't clash (but if you tried to use <= without qualifying it, you'd get an ambiguous reference error).

In the case of Rev, you get an error, though, because both the class method and the standalone function are declared in YourModule, which is illegal (multiple declarations of the same name).

So, long story short, go with the "instance" syntax.

On Wed, Jan 21, 2015 at 7:23 PM, Jeffrey Brown <jeffbrown.the@gmail.com> wrote:

Dear Haskellers,

The following compiles. (Rev stands for Reversible, and Dirn for Direction.)

    class Rev a where
        rev :: a -> a

    data Dirn = Succ | Pred
        deriving (Eq, Show, Ord)

    -- implement Ord
    (<=) Succ Pred = False
    (<=) _ _ = True

    -- implement Rev
    instance Rev Dirn where
        rev Succ = Pred
        rev Pred = Succ

But if I try to define the Rev instance the same way the Ord instance is being defined, it does not compile:

    class Rev a where
        rev :: a -> a

    data Dirn = Succ | Pred
        deriving (Eq, Show, Ord, Rev)

    -- implement Ord, because Dirn is used as a key in a Map
    (<=) Succ Pred = False
    (<=) _ _ = True

    -- implement Rev
    rev Succ = Pred
    rev Pred = Succ

What's going on?

Many thanks,
Jeff

_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://www.haskell.org/mailman/listinfo/beginners