Hmm. The feature in this paper looks really useful! Right now I am working on log writing/recovery code and running into the difficulty/annoyance of wanting to override the read/show methods for CalendarTime provided by the Time library. I don't want to have to write my own Read/Show for all the various intervening types that eventually contains CalendarTime! (Note: Based on my cursory read, I don't think I can use implicit parameters to solve this problem so if there is a choice between one or the other, I prefer named instances!) I'm not sure, but it also looks like names instances may give you the ability to declare Set as an instance of Monad. Yay! (apropos a prior thread). I can't tell whether it allows me to do the type defaulting that would have been useful for the DBMS code, but if it did, even better! Another nice thing about named instances is that it gets rid of the annoying lack of globality of instance declarations. For some reason it is ok to do: import MyModule (hiding myFunc) import MyModule2 (myFunc) But you can't do the same with instance declarations. Note: I don't know if this means that myFunc means different things in different places, but I don't see why instance declarations should be treated differently. -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com On Tue, 4 May 2004, Andre Pang wrote:
On 04/05/2004, at 6:39 PM, Simon Peyton-Jones wrote:
| For example, Network.URI should not have an | Eq instance because some applications may | legitimately want to define URI equivalence | differently (e.g. case sensitivity). OTOH, there | are lots of times when the default behavior is | just fine and having to define readsPrec/showsPrec | manually is just a PITA. | | If there isn't already a way to do this | how about this strawman syntax: | | instance Eq URI default
(Sorry, this message should be in reply to Alex Jacobson's post, but I jumped in on the discussion late and don't have the original message to reply to).
There's a paper by Wolfram Kahl and Jan Scheffczyk about "named instances" which targets that exact problem:
http://www.informatik.uni-bonn.de/~ralf/hw2001/4.html.
Although the functional programming language Haskell has a powerful type class system, users frequently run into situations where they would like to be able to define or adapt instances of type classes only after the remainder of a component has been produced. However, Haskell's type class system essentially only allows late binding of type class constraints on free type variables, and not on uses of type class members at variable-free types. In the current paper we propose a language extension that enhances the late binding capabilities of Haskell type classes, and provides more flexible means for type class instantiation. The latter is achieved via named instances that do not participate in automatic context reduction, but can only be used for late binding. By combining this capability with the automatic aspects of the Haskell type class system, we arrive at an essentially conservative extension that greatly improves flexibility of programming using type classes and opens up new structuring principles for Haskell library design. We exemplify our extension through the sketch of some applications and show how our approach could be used to explain or subsume other language features as for example implicit parameters. We present a typed lambda-calculus for our extension and provide a working prototype type checker on the basis of Mark Jones' ``Typing Haskell in Haskell''.
Of course, the changes made to the "Typing Haskell in Haskell" type checker would have to be propagated back to GHC (if that's possible).
-- % Andre Pang : trust.in.love.to.save