19 Jan
2009
19 Jan
'09
4:31 p.m.
On Mon, Jan 19, 2009 at 01:13:37PM -0800, Jonathan Cast wrote:
(On the other hand, your hunch that lift = return is correct --- so you get a cookie for that; it's just that return here is neither the return of the monad for m nor the return of the monad for ReaderT m. It is, instead, the return of the *applicative functor* --- on the category of monads and monad homomorphisms --- associated to the monad transformer ReaderT.)
It's also a monad in the category of monads, as are ErrorT, StateT and WriteT (see Moggi, An Abstract View of Programming Languages, 1989, s4).