new major release of transformers package

[Michael Snoyman]

On Thu, Mar 8, 2012 at 2:43 AM, Ross Paterson <ross at soi.city.ac.uk> wrote:
> Seeking views before a new major release of transformers package.
> The docs are here:
>
>        http://code.haskell.org/~ross/transformers/dist/doc/html/transformers/
>
> The source is here:
>
>        darcs get http://code.haskell.org/~ross/transformers
>
> The major changes from version 0.2.2.0 are:
>
> * Foldable and Traversable instances for transformers that support them.
> * extra Monad instances:
>
>        instance (MonadFix m) => MonadFix (MaybeT m)
>        instance (MonadFix m) => MonadFix (IdentityT m)
>        instance (Monad f, Monad g) => Monad (Product f g)
>        instance (MonadPlus f, MonadPlus g) => MonadPlus (Product f g)
>        instance (MonadFix f, MonadFix g) => MonadFix (Product f g)
>
> * new functors Backwards and Reverse
> * a new Lift transformer, a generalization of Errors
> * generalized constructor functions:
>
>        state :: Monad m => (s -> (a, s)) -> StateT s m a
>        reader :: Monad m => (r -> a) -> ReaderT r m a
>        writer :: Monad m => (a, w) -> WriterT w m a
>
> Another issue that has been raised is: should the instance
>
>        instance Monad (ContT r m)
>
> have a Monad constraint so that it can define fail?
>
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I doubt that this change could actually be merged into transformers,
since it requires either FunDeps or Type Families, but I thought I'd
mention it anyway. In Yesod, we have the monads Handler and Widget,
which are essentially:

newtype Handler a = Handler (ReaderT HandlerData IO a)
newtype Widget a = Widget (WriterT WidgetData Handler a)

We could in theory make the underlying monad a type variable as well,
but this would produce confusing type signatures and error
messages[1], as well as falsely give the impression that it would be
valid to use different monads as the base for each of these.

The result? We have something which is essentially a transformer, but
actually isn't. Therefore, even though we *want* to have a `lift`
function, we can't define a `MonadTrans` instance.

My solution was to create a new typeclass[2]:

class MonadLift base m | m -> base where
    lift :: base a -> m a

It's simple to automatically make all instances of MonadTrans an
instance of MonadLift:

instance (Monad m, MonadTrans t) => MonadLift m (t m) where
    lift = Control.Monad.Trans.Class.lift

and still make separate instances for Handler and Widget.

As I said, I'm not really trying to push this into transformers, but I
thought I would mention it. I think being able to make non-MonadTrans
transformers can often be a good API design, and it would be nice to
support it in the libraries.

Michael

[1] The type aren't quite as simple as I've presented them here.
[2] http://hackage.haskell.org/packages/archive/yesod-core/0.10.2.1/doc/html/Yesod-Handler.html#t:MonadLift