Re: [Haskell-cafe] specifying using type class

I don't know whether this is really applicable but: isn't emptyStack in Ertugrul last message some kind of constructor? You can add any kind of special constructors as functions in the type class which return a new queue. For example: class Stack s where newEmptyStack :: s a newSingletonStack :: a -> s a ... Why doesn't this fulfill you needs of specifying ways to construct new elements? 2012/7/23 Patrick Browne

On 22/07/12, *Ertugrul Söylemez * wrote:

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:

No really. I am investigating the strengths and weaknesses of type classes as a *unit of specification*. I am aware that their primarily intended to act as interface description, which I suppose is a form of specification. To what degree could the QUEUE_SPEC (repeated below) from my first posting be expressed as a type class? From the feedback, I get the impression that an abstract specification such as QUEUE_SPEC cannot be expressed as a type class (as an instance yes). The stumbling block seems to be the abstract representation of constructors. In [1] the classes Moveable and Named are combined, but again each of these classes are pure signatures.

Regards, Pat [1]Haskell: The Craft of Functional Programming (Second Edition) Simon Thompson, page 270

module QUEUE_SPEC where data Queue e = New | Insert (Queue e) e deriving Show

isEmpty :: Queue e -> Bool isEmpty New = True isEmpty (Insert q e) = False

first :: Queue e -> e first (Insert q e) = if (isEmpty q) then e else (first q)

rest :: Queue e -> Queue e rest (Insert q e ) = if (isEmpty q) then New else (Insert (rest q) e)

size :: Queue e -> Int size New = 0 size (Insert q e) = succ (size q)

{- some tests of above code size (Insert (Insert (Insert New 5) 6) 3) rest (Insert (Insert (Insert New 5) 6) 3)

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