...

On the other hand, it's difficult or impossible to make a list of a bunch of different types of things that have nothing in common save being members of the class.

I've recently been playing with making, for each class C, a "interface" datatype IC (appropriately universally and existentially qualified so as to include a dictionary for class C), and then making this IC an instance of class C. Then I can wrap any instance of C up in an IC, and make a list of those.

The casts get a bit annoying, though; the compiler can't figure out that this IC is in some sense the maximum type in class C, and so can't resolve things like

f :: MyClass a => [a] -> b f = ...

upcast :: MyClass a => a -> IMyClass -- usually defined as an instance of class Cast upcast x = IMyClass x

f [upcast a, upcast b] -- yields type error

Instead, you have to redefine f as follows:

f' :: [IMyClass] -> b

which is a bit annoying.

HTH.

--KW 8-)

Not surprisingly, the wiki (http://www.haskell.org/hawiki/ExistentialTypes) has some discussion about this as well, though not too much to add to what has been said.