Re: Fwd: transformers versus mtl

On Tue, Mar 24, 2009 at 4:39 PM, Isaac Dupree wrote:

Iavor Diatchki wrote:

...

On Tue, Mar 24, 2009 at 2:42 PM, Sittampalam, Ganesh

wrote:

...

Sittampalam, Ganesh wrote: ...the Functor instances for ReaderT r m and WriterT w m now (correctly) depend on Functor m, whereas in mtl they depend on Monad m.

...?On the other hand, with the unfortunate lack of relation between Functor and Monad in Haskell, it seems these instances will not work if I try to transform a monad that does not have a functor instance.

well, if base monad `m` is not a Functor, then it seems decidedly odd to me for the transformed monad to become a Functor... the transformer would be doing more than you'd expect!  But if you don't use (Functor (ReaderT r m)), only (Monad (ReaderT r m)), perhaps it works as well as could be expected? (Or maybe it doesn't work after all; then my logic here is useless)

-Isaac

I'm pretty sure all monads are functors. Alex