Adding IdentityT to mtl

[Donald Bruce Stewart]

Thanks Iavor!

iavor.diatchki:
> Hi,
> I comlpletely forgot about this.  I have added two new transformers to
> monadLib: IdT and LiftT, the second one using a strict bind.  These
> changes are available in the darcs repository.  When I get around to
> playing around with them a bit more I will make a new package and put
> it on hackage.
> -Iavor
> 
> On 8/28/07, Josef Svenningsson <josef.svenningsson at gmail.com> wrote:
> > 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 <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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