On Thursday 14 April 2011 14:25:39, Daniel Díaz wrote:
Hi,
I have a question about the context of the execWriterT function, in Control.Monad.Writer of the mtl package. The current context forces the containter type to be a Monad, while is enough a Functor. Yes, WriterT is a monad transformer, so this is not a problem, but I have as a law to write the less restrictive context for every function.
So, is there any reason to do this context more restrictive?
Alas, Functor is not a superclass of Monad, so a Monad constraint is not more restrictive than a Functor constraint. And e.g. in mtl-2.*, we have instance Functor m => Functor (Control.Monad.Trans.State.Lazy.StateT s m) which used to have a Monad constraint (same for WriterT), a change that broke some code (previously, one could have foo :: (Monad m) => ... StateT s m a ... foo -- uses the Functor instance , now you have to have foo :: (Monad m, Functor m) => ... StateT s m a ... if you use both, Functor and Monad, in foo).
If not, the obvious way to do it may be:
execWriterT :: Functor m => WriterT w m a -> m w execWriterT = fmap snd . runWriterT
However, I thought that there must be a reason explaining this fact.