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
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?
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