[Haskell-cafe] Context in data and class declarations (was haskellprogramming guidelines)

Brian Hulley brianh at metamilk.com
Sat Feb 25 08:23:39 EST 2006

Brian Hulley wrote:
> <snip>
> Another confusing thing is the use of the word "inheritance" in
> tutorials/books about class declarations. Unlike object oriented
> languages, where a class or interface gets all the methods of its
> ancestor classes/interfaces in addition to some new methods declared
> at that level, each Haskell type class is completely independent of
> any other type class. For example, the class Ord contains methods for
> (<) (<=) (>=) (>) max min but does not contain the methods of Eq even
> though this confusing word "inheritance" or "superclass" would imply
> that it should. Ord does *not* "inherit" anything at all - the
> meaning of the Eq context in the class declaration is just that we
> will need the Eq dictionary in addition to the Ord dictionary when
> calling any of Ord's methods.
> Thus I propose that the contexts don't really have any place in a
> class declaration either ie
>   class Eq a => Ord a where
>           (<), (<=), ... ::
> would be better written as:
>     class Ord a where
>           (<), (<=), .... :: Eq a => a->a->Bool
> so the language wouldn't be so confusing to learn. Classes are after
> all extremely simple! :-)

Sorry folks I've got all this completely wrong! ;-)
It seems the Ord dictionary does in fact contain the Eq dictionary since the 
following examples compile:

test :: Ord a => a->a->Bool
test x y = x <= y

test2 :: Ord a => a->a->Bool
test2 x y = x == y

I think what confused me was that although the Ord dictionary contains the 
methods for Eq, you cannot override the Eq methods in an instance 
declaration for Ord - you have to use two instance declarations to achieve 
this, one for Ord and one for Eq, so as far as instance declarations are 
concerned, the type classes are separate, but the dictionaries are not.

Regards, Brian. 

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