Re: Proposal: make liftF a method of MonadFree

What about having the class head be Monad m, Functor f=> … MonadFree f m .. ? Is the motivation here to have a more performant liftF? What are some examples of more efficient implementations for current instances and what’s the performance delta? On Fri, Jul 16, 2021 at 6:53 PM David Feuer wrote:

Another flavor would be to leave liftF alone and add a method that does the same thing with a different name. This would preserve performance characteristics for instances like FT, for situations where the current implementation is faster.

On Fri, Jul 16, 2021, 4:55 PM David Feuer wrote:

...

We have

class Monad m => MonadFree f m | m -> f where wrap :: f (m a) -> m a

liftF :: (Functor f, MonadFree f m) => f a -> m a liftF = wrap . fmap pure

I propose we change this to

class Monad m => MonadFree f m | m -> f where wrap :: f (m a) -> m a

liftF :: f a -> m a default liftF :: Functor f => f a -> m a liftF = wrap . fmap pure

and add a function

defaultWrap :: MonadFree f m => f (m a) -> m a defaultWrap = join . liftF

This change is not strictly backwards compatible. Some instances might, hypothetically, have to add a Functor constraint. For example, the classic Control.Monad.Free and Control.Monad.Trans.Free would need them. However, those instances already have (currently redundant) Functor constraints, so that doesn't seem like a big deal.

An alternative would be to hew more strictly to backwards compatibility by placing a Functor f constraint on liftF. This seems a bit sad for "freer" instances that don't need it. For example, we have

newtype FT f m a = FT { runFT :: forall r. (a -> m r) -> (forall x. (x -> m r) -> f x -> m r) -> m r }

for which

liftF :: f a -> FT f m a liftF fa = FT $ \pur bndf -> bndf pur fa

Pull request at https://github.com/ekmett/free/pull/208

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