I think you are confusing ADTs, type classes and default declarations in type classes. In Haskell, values are truly created only via abstract data types. That would be a specific instance of your class:
data Stack a = Empty | Top a (Stack a)
Then you can write the implementation with respect to this concrete example.
What I was proposing, if you only need constructs, is that instead of thinking of constructors, you may think of a "factory" pattern, similar to that pattern in OOP: a function that creates the element for you. That would be the "newStack" in your type class: every instance of a Stack must provide a way to create new objects. However, this is just a function, so you cannot pattern match there.
What type classes do allow you to do is to provide default implementations of some function in you type class. But this must be a complete implementation (I mean, executable code), not merely a specification of some properties. For example, say you have functions in your class for testing emptiness and poping elements. Then you may write a default implementation of length:
class Stack s a | s -> a where
isEmpty :: s a -> Bool
pop :: s a -> (a, s a) -- Returns the top element and the rest of the stack
length :: s a -> Int
length stack = if isEmpty stack then 0 else (length (snd (pop stack))) + 1
However, I think that what you are trying to do is to encode some properties of data types into the type system. This is better done using "dependent typing", which in a very broad sense allows you to use value functions into types. For example, you may encode the number of elements of a stack in its type (so the type would be Stack <ElementType> <NumberOfElements>) and then you may only pop when the Stack is not empty, which could be encoded as
pop :: Stack a (n + 1) -> (a, Stack a n)
Hope this helps!
2012/7/23 Patrick Browne
<patrick.browne@dit.ie>
Yes. I tried that, but due to my lack of Haskell expertise I cannot write the constructor insertC1 below:
class QUEUE_SPEC_CLASS1 q where
newC1 :: q a
isEmptyC1 :: q a -> Bool
insertC1 :: q a -> a -> q a
sizeC1 :: q a -> Int
restC1 :: q a -> q a
-- I do not know how to specify constructor insertC1 ?? = ??
insertC1 newC1 a = newC1
isEmptyC1 newC1 = True
-- isEmpty (insertC1 newC1 a) = False
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
<patrick.browne@dit.ie <patrick.browne@dit.ie>>
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)
Tá an teachtaireacht seo scanta ó thaobh ábhar agus víreas ag Seirbhís Scanta Ríomhphost de chuid Seirbhísí Faisnéise, ITBÁC agus meastar í a bheith slán.
http://www.dit.ie
This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean.
http://www.dit.ie
_______________________________________________
Haskell-Cafe mailing list
http://www.haskell.org/mailman/listinfo/haskell-cafe
Tá an teachtaireacht seo scanta ó thaobh ábhar agus víreas ag Seirbhís Scanta Ríomhphost de chuid Seirbhísí Faisnéise, ITBÁC agus meastar í a bheith slán.
http://www.dit.ie
This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean.
http://www.dit.ie