Question

I defined a data type:

Data Card = Card Int deriving (Show, Eq)

(I defined also a type synonym:

Type Cards = [Card]

)

and then make it instance of:

 instance Ord Card where
    x > y   |ix == iy   = False
            |ix == 0    = True
            |iy == 0    = False
            |otherwise  = (ix > iy)
        where 
            ix = label x
            iy = label y

Then when I type:

 (Card x) > (Card y) :: x,y are Int 

it works, but when I type:

 [(Card x)] > [(Card y)] :: x,y are Int 

it goes in loop.

Why does it happen? How do I fix it?

Answer 1

Your Ord instance for Card only defines >, but you need to define either <= or compare:

Minimal complete definition: either compare or <=.

The Ord instance for lists defines compare in terms of the underlying data type’s compare. By default compare and <= are defined in terms of each other, hence they don’t terminate when called if you have not defined either of them. The other Ord operations for list (including >) are defined in terms of compare, hence why > doesn’t terminate when you call it on [Card].