Dear all, Ross Peterson wrote:

The favourite customer for FDs has been the monad transformer library. ... What other libraries should Haskell' support, and what are their requirements?

Here are some classes from Yampa/earlier versions of FRP. I shouldn't think they're particularly demanding. Also, I'm not saying these classes could not be defined differently/better. They are just examples of what seems to me reasonable uses of FDs. --------------------------------------------------------- -- Minimal instance: zeroVector, (*^), (^+^), dot class Floating a => VectorSpace v a | v -> a where zeroVector :: v (*^) :: a -> v -> v (^/) :: v -> a -> v negateVector :: v -> v (^+^) :: v -> v -> v (^-^) :: v -> v -> v dot :: v -> v -> a norm :: v -> a normalize :: v -> v ---------------------------------------------------------- -- Minimal instance: origin, .+^, .^. class (Floating a, VectorSpace v a) => AffineSpace p v a | p -> v, v -> a where origin :: p (.+^) :: p -> v -> p (.-^) :: p -> v -> p (.-.) :: p -> p -> v distance :: p -> p -> a ---------------------------------------------------------- From an old version of FRP: FRPCore.lhs:> class MixSwitchable s a b | s a -> b where FRPCore.lhs:> class Switchable s i | s -> i where FRPCore.lhs: class RunningIn a b i | a -> i where FRPCore.lhs:> class ImpAs a b | a -> b where FRPTask.lhs: class RunningInTask a t i | a t -> i where FRPTask.lhs:> class Monad m => StateMonad s m | m -> s where FRPTask.lhs:> class Monad m => EnvMonad env m | m -> env where FRPTask.lhs:> class GTask t => MsgTask t m | t -> m where FRPTask.lhs:> class MsgTaskMap mt m nt n | mt -> m, nt -> n where /Henrik -- Henrik Nilsson School of Computer Science and Information Technology The University of Nottingham nhn@cs.nott.ac.uk This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

What other libraries should Haskell' support, and what are their requirements?

useful initiative! will your collection be available anywhere? may I suggest that you (a) ask on the main Haskell and library lists for better coverage (I would have thought that the alternative Num prelude suggestions might have some use cases), and (b) collect non-use cases as well (eg, where current implementations are buggy/incomplete/do different things, or where other reasons have prevented Haskellers from using FDs so far)? I think trying to clean up the latter will be more effective than wading through dozens of variations of the same working examples - you're looking for counter-examples to the current design, aren't you? and just in case you haven't got these on your list already, here are some examples from earlier discussions on this mailing list: - ticket #92 has module Data.Records attached to it. http://hackage.haskell.org/trac/haskell-prime/ticket/92 I'd like to be able to use that in Haskell'. the library is useful in itself (I've used its record selection and concatenation parts when encoding attribute grammars), and I also suggested it as a good testcase for Haskell' providing a sufficient (but cleaned-up) subset of currently available features. but it is also an example of code that - works with GHC, but not with Hugs; one of those problems I reported on hugs-bugs: http://www.haskell.org//pipermail/hugs-bugs/2006-February/001560.html and went through a few of the Hugs/GHC differences here on this mailing list: http://www.haskell.org//pipermail/haskell-prime/2006-February/000577.html and used the Select example to motivate the need for relaxed coverage in termination checking: http://www.haskell.org//pipermail/haskell-prime/2006-February/000825.html I have since come to doubt that GHC really solves the issue, it just happens that its strategy of delaying problems until they may no longer matter works for this example; but one can construct other examples that expose the problem in spite of this delayed complaining trick. see my own attempts to show FD problems here: http://www.haskell.org//pipermail/haskell-prime/2006-February/000781.html or Oleg's recent example on haskell-cafe: http://www.haskell.org//pipermail/haskell-cafe/2006-April/015372.html while I didn't quite agree with his interpretation (see my answer to his message), he did manage to construct an example in which GHC accepts a type/program in violation of an FD. - requires complex workarounds, thanks to current restrictions, where the same could be expressed simply and directly without; (compare the code for Remove in Data.Record.hs: the one in comments vs the one I had to use to make GHC happy) - things like a simple type equality predicate at the type class level run into problems with both GHC and Hugs. reported to both GHC and Hugs bugs lists as: http://www.haskell.org//pipermail/hugs-bugs/2006-February/001564.html - the FD-visibility limitations strike not only at the instance level. here is a simplified example of a problem I ran into when trying to encode ATS in FDs (a variable in a superclass constraint that doesn't occur in the class head, but is determined by an FD on the superclass constraint): http://hackage.haskell.org/trac/ghc/ticket/714 - the HList library and associated paper also use and investigate the peculiarities of FDs, and variations on the TypeEq theme (it has both unpractical/portable and practical versions that make essential use of some limitations in GHC's type class implementation to work around other of its limitations; it demonstrates wonderfully why the current story needs to be cleaned up!): http://homepages.cwi.nl/~ralf/HList/ hope that's the kind of thing you are looking for?-) cheers, claus