Documentation

The Ord class is used for totally ordered datatypes.

Instances of Ord can be derived for any user-defined datatype whose constituent types are in Ord. The declared order of the constructors in the data declaration determines the ordering in derived Ord instances. The Ordering datatype allows a single comparison to determine the precise ordering of two objects.

Ord, as defined by the Haskell report, implements a total order and has the following properties:

Comparability

x <= y || y <= x = True

Transitivity

if x <= y && y <= z = True, then x <= z = True

Reflexivity

x <= x = True

Antisymmetry

if x <= y && y <= x = True, then x == y = True

The following operator interactions are expected to hold:

1. x >= y = y <= x
2. x < y = x <= y && x /= y
3. x > y = y < x
4. x < y = compare x y == LT
5. x > y = compare x y == GT
6. x == y = compare x y == EQ
7. min x y == if x <= y then x else y = True
8. max x y == if x >= y then x else y = True

Note that (7.) and (8.) do not require min and max to return either of their arguments. The result is merely required to equal one of the arguments in terms of (==). Users who expect a stronger guarantee are advised to write their own min and/or max functions.

The nuance of the above distinction is not always fully internalized by developers, and in the past (tracing back to the Haskell 1.4 Report) the specification for Ord asserted the stronger property that (min x y, max x y) = (x, y) or (y, x), or in other words, that min and max will return one of their arguments, using argument order as the tie-breaker if the arguments are equal by comparison. A few list and Foldable functions have behavior that is best understood with this assumption in mind: all variations of minimumBy and maximumBy (which can't use min and max in their implementations) are written such that minimumBy compare and maximumBy compare are respectively equivalent to minimum and maximum (which do use min and max) only if min and max adhere to this tie-breaking convention. Otherwise, if there are multiple least or largest elements in a container, minimum and maximum may not return the same one that minimumBy compare and maximumBy compare do (though they should return something that is equal). (This is relevant for types with non-extensional equality, like Arg, but also in cases where the precise reference held matters for memory-management reasons.) Unless there is a reason to deviate, it is less confusing for implementors of Ord to respect this same convention (as the default definitions of min and max do).

Minimal complete definition: either compare or <=. Using compare can be more efficient for complex types.