On Wed, Apr 12, 2006 at 03:57:31PM +0100, Henrik Nilsson wrote:
Here are some classes from Yampa/earlier versions of FRP.
The instances are probably more important, especially instances that violate the following restriction (from the original description of FDs, christened the "coverage condition" in the FD-CHR paper): any type variable occurring in a range argument must also occur in a domain argument. The following (from mtl) satisfies this condition (and is therefore boring): class (Monoid w, Monad m) => MonadWriter w m | m -> w instance (Monoid w, Monad m) => MonadWriter w (WriterT w m) (because w occurs in WriterT w m) but this (also from mtl) does not: instance (Error e, MonadWriter w m) => MonadWriter w (ErrorT e m) Here w is determined by ErrorT e m, via the FD on the context. GHC and Hugs use this laxer notion of dependency, but it has emerged that this system is not well-behaved, so it has been proposed that we use some intermediate conditions on instances. One possibility is: Instances must either satisfy the coverage condition, or 1) the functional dependency must be full, i.e. involve all arguments of the class, and 2) the range arguments of an instance must be distinct type variables determined by domain variables, but not occurring elsewhere in the instance head. That's pretty clunky, but it handles the mtl instances. Does it permit enough of the uses people want?
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
The dependencies here are not full, so this is likely to be problematic. Presumably you can't split the class because you want the default definition of distance to use (.-.) and norm.
FRPCore.lhs: class RunningIn a b i | a -> i where FRPTask.lhs:> class MsgTaskMap mt m nt n | mt -> m, nt -> n where
Ditto