On 22/07/12, Ertugrul Söylemez <es@ertes.de <es@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 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)