Adding IdentityT to mtl

[Iavor Diatchki]

Hi,

Just curious, could you say a bit more about what you plan to use this for?

By the way, if IdentityT is added, then it should have instances for
all the classes---Reader,Writer,etc.   Also the Functor instance does
not need the Monad constraint on "m".

-Iavor


On 5/31/07, Donald Bruce Stewart <dons at cse.unsw.edu.au> wrote:
> I wanted an IdentityT today, for extending xmonad. (The idea is to
> allower user-defined monad transformers, so users can plug in their own
> semantics easily).
>
> By default it would use IdentityT, which I note is not in mtl!
>
> Here's roughly what it would be:
>
>     -----------------------------------------------------------------------------
>     -- |
>     -- Module      :  Identity.hs
>     -- License     :  BSD3-style (see LICENSE)
>     --
>     module IdentityT where
>
>     import Control.Monad.Trans
>
>     --
>     -- IdentityT , a parameterisable identity monad, with an inner monad
>     -- The user's default monad transformer
>     --
>
>     newtype IdentityT m a = IdentityT { runIdentityT :: m a }
>
>     instance (Functor m, Monad m) => Functor (IdentityT m) where
>         fmap f = IdentityT . fmap f . runIdentityT
>
>     instance (Monad m) => Monad (IdentityT m) where
>         return   = IdentityT . return
>         m >>= k  = IdentityT $ runIdentityT . k =<< runIdentityT m
>         fail msg = IdentityT $ fail msg
>
>     instance (MonadIO m) => MonadIO (IdentityT m) where
>         liftIO = IdentityT . liftIO
>
> Any reasons why this shouldn't be in mtl?
>
> -- Don
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