list monad transformer

[Ralf Hinze]

> when you talk about a backtracking monad transformer which one are you
> refering to?  ralf hinze seems to have a whole bunch of papers on the
> matter, some of which seem to be a lot like the "Maybe" transformer and
> some seem very complicated (but presumable are very efficient).

A simple one is this

	type CPS a ans                =  (a -> ans) -> ans

	newtype BacktrT m a           =  B (forall ans . CPS a (m ans -> m ans))

Cheers, Ralf