Ord – ordered types
The Ord class requires any type that wants to be an instance of it to first be an instance of the Eq class by using a class constraint, and additionally, to support the following methods:
class (Eq a) => Ord a where
(<), (<=), (>), (>=) :: a -> a -> Bool
min, max :: a -> a -> a
In other words, the Ord class extends the Eq class and supports additional methods (<), (<=), (>), (>=), min and max. The min and max methods are defined by default as:
min x y
| x <= y = x
| otherwise = y
max x y
| x <= y = y
| otherwise = x
And for a minimal definition of the class, we just need to define the (<=) method because the other ones also have default definitions:
class (Eq a) => Ord a where
(<), (<=), (>), (>=) :: a -> a -> Bool
min, max :: a -> a -> a
​
-- Minimal complete definition:
-- (<=)
x < y = x <= y && x /= y
x > y = y < x
x >= y = y <= x
min x y
| x <= y = x
| otherwise = y
max x y
| x <= y = y
| otherwise = x
All the basic types of Haskell are also instances of the Ord class.
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