2011/12/30 Stanisław Findeisen

Could anyone please explain to me what is going on here?

You must, when declaring an instance of Ord, provide *either* a definition of (compare) or a definition of (<=). Both have default implementations defined, but in terms of each other; so if you don't provide either one in your instance declaration, you get an infinite loop when you try to use Ord methods (because (compare) invokes (<=), which invokes (compare) ...). -- brandon s allbery allbery.b@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms

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

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