With lots of rejoicing, today I discovered that the base-3.0 library, to be distributed with the upcoming GHC 6.10, will contain the Control.Category type class. This is exactly what is needed to put Thrist (~>) into a general context!
Can't wait till September...
PS: It is also a nice surprise that Ashley Yakeley, the maintainer of this module is an old-time Dylan person :-)
Sometime I have to blog about the lots of developers who share a common Haskell/Dylan heritage.