[Haskell-cafe] What I wish someone had told me...

[Daniel Fischer]

Am Mittwoch, 15. Oktober 2008 00:34 schrieb Derek Elkins:
> It's not technically true.  Type classes and interfaces a la Java are
> very fundamentally different neither is remotely capable of doing what
> the other does.  

Could you elaborate on that, please?
I always understood Java's interfaces to be somewhat similar to type classes 
(and I learnt the bit of Java I know before I even knew the term "Functional 
Programming", never really got the whole OO thing though).
An interface, I thought, is a contract stating that the classes implementing 
that interface provide certain operations (obeying some rules). If there's 
more to interfaces, I'm happily unaware of that :)
In what way is that "very fundamentally different" from type classes?

As the languages as a whole are fundamentally different, that similarity is of 
course rather superficial, so I will not say that stressing it is beneficial, 
but I'm not convinced it is detrimental either.

>
> I strongly agree with the thrust of your email.  This "type classes are
> kinda like interfaces" meme is horrible.  

Because of the tendency to confuse OO programmers learning Haskell, or is 
there a deeper reason? (Answer to this question could be superfluous after 
addressing the above)

>  For example, this
> is a line from RWH: "Typeclasses may look like the objects of
> object-oriented programming, but they are truly quite different."  Also
> later there is another sidebar along those lines.
>

Now that is something I wouldn't have dreamt of.
If anything, I would relate objects to values (except that objects tend to be 
mutable).

Thanks,
Daniel