Haskell naming conventions

[Fergus Henderson]

On 23-Dec-2003, Sean L. Palmer <seanpalmer at verizon.net> 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 <fjh at cs.mu.oz.au>  |  "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.