The problem with interfaces as a replacement for type classes is that they only provide dispatch based on the specific type of the first argument (i.e. the receiver).
 
Type classes allow you to dispatch based on return type, and on the instantiations of generic parameters. Neither of these things is reasonably possible with interfaces.
 
For example you can't directly implement the Read type class with interfaces. Neither can you implement a function of type [a] -> ... where the dispatch is based on the instantiation of a - even if you can add an interface to the [] generic type, you might not have a concrete object of type a to dispatch from if the empty list is passed as an argument.
 

From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Peter Verswyvelen
Sent: 13 January 2010 09:52
To: Gregory Collins
Cc: haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] How to fulfill the "code-reuse" destiny of OOP?

On Sun, Nov 1, 2009 at 2:57 AM, Gregory Collins <greg@gregorycollins.net> wrote:
Doing OO-style programming in Haskell is difficult and unnatural, it's
true (although technically speaking it is possible). That said, nobody's
yet to present a convincing argument to me why Java gets a free pass for
lacking closures and typeclasses.

I might be wrong, but doesn't Java's concepts of inner classes and interfaces together with adapter classes can be used to replace closures and typeclasses in a way?

An inner class allows you to implicitly capture the parent object ("environment"), just like a closure does in a sense.

Interfaces group together methods, like type classes do. 

Although I'm actually a C# fanboy for doing "industrial" programming, I think the Java designers did an excellent job, finding a good balance in language features, ease of use and readability, and although C# does offer closures and many more FP constructs, I really miss the above Java constructs.



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