Fwd: transformers versus mtl

Iavor Diatchki iavor.diatchki at gmail.com
Tue Mar 24 19:14:38 EDT 2009

(Ganesh, sorry for the double post, I meant to send this to the list)


On Tue, Mar 24, 2009 at 2:42 PM, Sittampalam, Ganesh
<ganesh.sittampalam at credit-suisse.com> 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.


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