mtl: Context of execWriterT

[Daniel Díaz]

 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?

 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.