[Haskell-cafe] MonadTrans lift implementation

Ross Paterson ross at soi.city.ac.uk
Mon Jan 19 16:31:01 EST 2009

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).

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