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 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 _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

Whatever happened to the suggestion of extending mtl with IdentityT? I think it's reasonable, especially since we have a documented use case. /Josef On 6/1/07, Donald Bruce Stewart 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 _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries