[Haskell-cafe] Haskell's type system compared to CLOS

[Michael Vanier]

Matthias-Christian Ott wrote:
> Hi,
> usually I'm sceptical of programming languages which are not based
> on the von Neumann architecture, but recently I got interested in
> functional programming languages.
>
> The arrogance of lots of Haskell users, who made me feel that using a
> programming language other than Haskell is a waste of time, combined
> with vanguard mathematical notation has been very attractive to me
> and I have decided to get at least an idea of Haskell and its concepts.
>
> Some weeks ago I learned programming in Dylan and was impressed by its
> object system which is basically a stripped version of CLOS. Multiple
> dispatch together with a well-thought-out object system is quite
> powerful, because it removes the burden of including methods in the
> class definition.
>
> At the moment I'm reading the "Functional Programming using Standard
> ML" and I'm in the chapter on data types.
>
> This afternoon it occurred to me that classes and data types are
> symmetric. In a class hierarchy you start an abstract super class
> (the most abstract is the class object in some languages) and further
> specialise them via inheritance; with data types you can start with
> specialized versions and abstract them via constructors (I'm not sure
> how message sending to a superclass looks like in this analogy).
>
> Anyhow, I also came across an interesting presentation. Andreas Löh
> and Ralf Hinze state in their presentation "Open data types and open
> functions" [1]:
>
>     * OO languages support extension of data, but not of functionality.
>     * FP languages support extension of functionality, but not of data.
>
> Their first point refers to the fact that in most object-oriented
> languages don't allow the separate definition of classes and their
> respective methods. So to add new functions, you have edit the class
> definitions.
>
> However, in functional programming languages you can easily add new
> functionality via pattern matching, but have to either introduce new
> types or new constructors, which again means to modify existing code.
>
> In Dylan (and in Common Lisp) you define methods separate from classes
> and have pattern matching based on types. This solves all mentioned
> problems.
>
> So my question is, how are algebraic data types in Haskell superior to
> CLOS (despite the fact that CLOS is impure)? How do both compare?
>
> What has Haskell to provide what Common Lisp and Dylan haven't?
>
> Thanks!
>
> Regards,
> Matthias-Christian
>
>
>   
Type classes.

Mike