Henning Thielemann wrote:
Sittampalam, Ganesh wrote:
Henning Thielemann wrote:
What about adding the Maybe monad transformer as provided in MaybeT package to the transformers package?
The module name could be Control.Monad.Trans.Maybe which would also avoid conflicts with the MaybeT package.
What MonadPlus instance will it get, if any?
The one which is currently used by MaybeT:
instance (Monad m) => MonadPlus (MaybeT m) where mzero = MaybeT (return Nothing) mplus x y = MaybeT $ do v <- runMaybeT x case v of Nothing -> runMaybeT y Just _ -> return v
It is also the one I needed recently.
I tend to agree that this is the right one to use, but I think care should be taken when adding this to what is/is expected to be a widely used package, as it could make it difficult for people who want the alternate behaviour, and it might imply a degree of standardisation/ canonicity that is not really justified. On the other hand, perhaps there are no real uses for the other one. Cheers, Ganesh ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================