Re: Fwd: transformers versus mtl

On Tue, Mar 24, 2009 at 04:14:38PM -0700, 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.

Does anyone ever use ReaderT and WriterT with a type that is not a monad (i.e., use them as functor transformers rather then monad transformers)? 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.

It's more that they're used polymorphically, and the constraint Monad m is now insufficient: Functor m is also needed.