a universal printer for Haskell?

Johan Jeuring johanj@cs.uu.nl
Wed, 20 Feb 2002 16:29:26 +0100

>> You don't need meta-programming technology (reflection) to do things 
>> like
>> generic prinitng. A generic programming extension of Haskell (like
>> Generic Haskell, or derivable classes) can do the job for you.
> Isn't generic programming usually based on a kind of compile-time
> reflection (if the argument is of this type, do this, else..)?

Yes, you might view it as compile-time reflection. Programs
are still typable.

> And don't you write generic functions with the intention that the
> implementation will figure out the approriate type-specific
> variants, i.e., you write your code at a level of abstraction that
> is not easily reached with the standard language tools -- a meta
> level, where functions at different types are the first-class
> objects of discourse?

If you view that as a metalevel I agree. However, it is the intention
you never see the generated functions at different types: the compiler
can (in priciple) completely hide those from the programmer. In that
sense there is just one level: Generic haskell offers you normal
Haskell functions and generic functions.

> I find it helpful to think of generic programming support as one way
> of integrating well-behaved subsets of reflection and other
> meta-programming techniques into Haskell.
> It is partly a trade-off: you get some of the features and avoid
> some of the problems of a fully reflective architecture. It is also
> a specialisation: by avoiding the full generality, the specific
> subset of features can be designed in a structured fashion, with the
> application domain in mind, making them easier to use for that domain.


-- Johan