On Wed, Jan 13, 2010 at 7:57 AM, Peter Verswyvelen wrote:

A while ago, someone provided me a link to the paper "Type Inference for Overloading without Restrictions" http://www.dcc.ufmg.br/~camarao/ct-flops99.ps.gzhttp://www.dcc.ufmg.br/%7Ecamarao/ct-flops99.ps.gz

Although I don't understand everything in this paper, I wander what people's opinions are about this regarding a future Haskell language revision or extension?

System CT (as it was defined in the Flops99 paper) is a core system for constrained (parametric + ad-hoc) polymorphism, the main difference with respect to Haskell type classes being that it uses a closed world approach to overloading (as it was defined in the Flops99 paper), while Haskell type classes uses an open world. Open means that principal types of overloaded symbols are explicitly specified (in Haskell class declarations), and, informally speaking, satisfiability checking is performed only when overloading is resolved (i.e. satisfiability is delayed until it cannot be delayed any longer). Closed, on the other hand, means that principal types are determined (inferred) according to (definitions available in) the context (as occurs in the normal case, of non-overloaded symbols). For details, see e.g. [1]. A "mixed" approach, where annotation of types of overloaded symbols is not mandatory but satisfiability checking is done as in an open world approach, is possible and advantageous. Only prototype implementations exist to this date of system CT. A prototype implementation of a mixed approach is currently under way.

Would a feature like this be preferable over typeclasses?

The main disadvantage of system CT's closed world approach is the cost of satisfiability checking. (However, this does not hold in the case of a mixed approach, where satisfiability checking is restrictd as in an open world). The main advantage is that annotations (type classes) need not be mandatory and a priori determined by the programmer. Would it be practical to implement? Yes, I think so, in particular the mixed approach where satisfiability checking is delayed (it is done only if there exist "unreachable variables" in the constraint set), as explained in [2]. We have recently sent a message with a link to our paper, that describes this approach, as a proposal to solve Haskell's MPTC dilemma (without the need of functional dependencies or another mechanism).

Are people working on this?

Yes. We are.

Thanks, Peter Verswyvelen _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Cheers, Carlos [1] Open and Closed World Approaches to Overloading: a Definition and Support for Coexistence, Carlos Camarão, Cristiano Vasconcellos, Lucília Figueiredo, João Nicola, Journal of Universal Computer Science 13(6), 854-873, 2007. http://www.dcc.ufmg.br/~camarao/CT/open-closed.pdf [2] A Solution to Haskell's Multi-paramemeter Type Class Dilemma, Carlos Camarão, Rodrigo Ribeiro, Lucília Figueiredo, Cristiano Vasconcellos, SBLP'2009 (13th Brazilian Symposium on Programming Languages), 2009. http://www.dcc.ufmg.br/~camarao/CT/solution-to-mptc-dilemma.pdf