As you can probably see, there are circular definitions with compare and <= To make 'yourtype' an an instance of ord you must supply
-- Minimal complete definition: either 'compare' or '<='.
and then the circular definitions will be broken and your definition will provide the key missing part. br On 30 Dec 2011, at 11:51, Stanisław Findeisen wrote:
Hi
Could anyone please explain to me what is going on here?
------------------------------------------------------------------------ -- | 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. -- -- Minimal complete definition: either 'compare' or '<='. -- Using 'compare' can be more efficient for complex types. -- class (Eq a) => Ord a where compare :: a -> a -> Ordering (<), (<=), (>), (>=) :: a -> a -> Bool max, min :: a -> a -> a
compare x y = if x == y then EQ -- NB: must be '<=' not '<' to validate the -- above claim about the minimal things that -- can be defined for an instance of Ord: else if x <= y then LT else GT
x < y = case compare x y of { LT -> True; _ -> False } x <= y = case compare x y of { GT -> False; _ -> True } x > y = case compare x y of { GT -> True; _ -> False } x >= y = case compare x y of { LT -> False; _ -> True }
-- These two default methods use '<=' rather than 'compare' -- because the latter is often more expensive max x y = if x <= y then y else x min x y = if x <= y then x else y
http://www.haskell.org/ghc/docs/latest/html/libraries/base/src/GHC-Classes.h... ------------------------------------------------------------------------
AFAIU this is the definition of the Ord type class in ghc. But what is this <= function that is used in the definition of compare? Here:
else if x <= y then LT
-- This e-mail address is invalid, see: http://people.eisenbits.com/~stf/public-email-note.html .
OpenPGP: E3D9 C030 88F5 D254 434C 6683 17DD 22A0 8A3B 5CC0
_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners