[Haskell-cafe] Join a transformer

[Ross Paterson]

On Thu, Mar 14, 2013 at 05:24:23AM +0200, Michael Snoyman wrote:
>    I'm wondering if this pattern exists and has a name. We have the
>    concept of joining a Monad:
>    join :: Monad m => m (m a) -> ma
>    How about joining a monad transformer?
>    joinT :: (Monad m, MonadTrans t) => t (t m) a -> t m a

This is a monad in the category of monads.  Moggi discusses them in
"An Abstract View of Programming Languages", including which transformers
have joinT.  I was thinking of adding the class to the transformers package.