Add retractT about free HOT 8 CLOSED

It is currently possible to define

retractT :: (Functor (t m), Monad (t m), MonadTrans t, Monad m) =>
    FreeT (t m) m a -> t m a
retractT = iterTM join

Would you like this implementation?

from free.

That looks right.

from free.

Hm. There appears to be also a different version:

retractM :: (Functor m, Monad m) => FreeT m m a -> m a
retractM = iterT join

With that

retractT = retractM . hoistFreeT lift

Alternatively

retractM = runIdentityT . retractT . transFreeT IdentityT

Though I think iterT* join would be a better implementation for retract*.

Would you like both retractM and retractT?

from free.

I'd check to see which matches the semantics of concats from Pipes.Group, but I think that is (obviously) the former.

retractM is kinda boring though. I think just doing retractT might offer a path to reduce the amount of pipes-specific code that @Gabriel439 has to write. ;)

from free.

Oh wow, I didn't even realize there was an iterTM. I'm probably going to keep the pipes-group code the way it is just because it helps people understand how to write their own folds.

from free.

Having both a more general function and the example of fold code would be great. Perhaps the current concats code could be moved to a comment or the tutorial, and then let concats = retractT just like maps = transFreeT?

from free.

Alright, that sounds good. I can include the current implementation as a comment alongside the retractT version.

from free.

Added in patch b75fbf5

from free.