[Haskell-cafe] specifying using type class

[Ertugrul Söylemez]

Hi there Patrick,

Patrick Browne <patrick.browne at dit.ie> wrote:

> Thanks for you very clear explanation.
> Without committing to some concrete representation such as list I do
> not know how to specify constructors in the class (see below). As you
> point out a class may not be appropriate for an actual application,
> but I am investigating the strengths and weaknesses of class as a unit
> of *specification*. Regards, Pat
>
> -- Class with functional dependency
> class QUEUE_SPEC_CLASS2 a q | q -> a where
>    newC2 :: q a -- ??
>    sizeC2  :: q a -> Int
>    restC2  :: q a -> Maybe (q a)
>    insertC2 :: q a -> a -> q a
> -- Without committing to some concrete representation such as list I
> do not know how to specify constructor for insertC2 ?? =  ??
> insertC2  newC2 a = newC2 -- wrong isEmptyC2  :: q a -> Bool
>    isEmptyC2 newC2  = True
> --   isEmptyC2 (insertC2 newC2 a) = False wrong

You are probably confusing the type class system with something from
OOP.  A type class captures a pattern in the way a type is used.  The
corresponding concrete representation of that pattern is then written in
the instance definition:

    class Stacklike s where
        emptyStack :: s a
        push       :: a -> s a -> s a
        rest       :: s a -> Maybe (s a)
        top        :: s a -> Maybe a

        pop :: s a -> Maybe (a, s a)
        pop s = liftA2 (,) (top s) (rest s)

    instance Stacklike [] where
        emptyStack = []
        push       = (:)
        top        = foldr (\x _ -> Just x) Nothing
        rest []          = Nothing
        rest (Push _ xs) = Just xs

    data MyStack a = Empty | Push a (MyStack a)

    instance Stacklike MyStack where
        emptyStack     = Empty
        push           = Push

        top Empty      = Nothing
        top (Push x _) = Just x

        rest Empty       = Nothing
        rest (Push _ xs) = Just xs


Greets,
Ertugrul

-- 
Not to be or to be and (not to be or to be and (not to be or to be and
(not to be or to be and ... that is the list monad.
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