[Haskell-cafe] specifying using type class
Alejandro Serrano Mena
trupill at gmail.com
Mon Jul 23 11:31:13 CEST 2012
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
2012/7/23 Patrick Browne <patrick.browne at dit.ie>
> On 22/07/12, *Ertugrul Söylemez * <es at ertes.de> 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
> In  the classes Moveable and Named are combined, but again each of
> these classes are pure signatures.
> 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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