On 23-Dec-2003, Sean L. Palmer <[EMAIL PROTECTED]> wrote:
> It occurs to me that Haskell would be quite a bit easier for OO and
> traditional programmers to grasp if Haskell would actually use the
> correct, or at least more commonly used, names for things.
> For instance,
>
> data Maybe a = Nothing | Just a
>
> Maybe is a type constructor and Nothing and Just are data constructors.
>
> So it makes me wonder why the use of the data keyword... wouldn't it make more sense
> to say:
>
> type Maybe a = Nothing | Just a
>
> ?
I agree. That's one of the reasons why Mercury uses the word "type"
rather than "data" when declaring discriminated union types.
The corresponding declaration in Mercury is
:- type maybe(A) ---> nothing ; just(A).
> Likewise with class, type class, and instance:
>
> class Eq a where
> (==) :: a -> a -> Bool
>
> That actually declares a type class, not a class. So why the use of
> the keyword class? Is it done merely to confuse C++ and Java programmers?
This is indeed potentially confusing for programmers familiar with
C++/Java/C#/Ada/etc.
In Mercury, we adopted Haskell-style type classes, using semantics
very similar to those in Haskell. But we did adapt the syntax a bit.
In particular, we use "typeclass" rather than "class" for type class
declarations. So in Mercury, the corresponding declaration would look
like this:
:- typeclass eq(A) where [
func '=='(A, A) = bool
].
> The concept of type class in Haskell apparently roughly corresponds
> to the concept of "interface" in Java. So why not call it interface?
Haskell interfaces are really quite a bit more general than Java interfaces,
particularly when you consider common extensions such as multi-parameter
type classes, constructor classes, and functional dependencies. So I think
it would not help to use the word "interface".
--
Fergus Henderson <[EMAIL PROTECTED]> | "I have always known that the pursuit
The University of Melbourne | of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh> | -- the last words of T. S. Garp.
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