Ertugrul Söylemez wrote:
For another the same can be achieved with free monads in a more transparent and flexible manner:
import Control.Monad.Free.Class
data PromptF a b x = PromptF (b -> x) a deriving (Functor)
More accurately,
data PromptF' p x = forall a. PromptF (a -> x) (p a)
(with the obvious Functor instance).
Are we talking about the same PromptT here? I was assuming this one: https://hackage.haskell.org/package/prompt-0.1.1.2/docs/Control-Monad-Prompt....
I've looked a bit more closely at the difference between the prompt and MonadPrompt packages. PromptT a b t r from the prompt package is a newtype for forall m. Monad m => (a -> m (t b)) -> m (t r) using the "universal" Cont/Codensity monad, this becomes forall x. (a -> (t b -> x) -> x) -> (t r -> x) -> x) which is quite similar to the Prompt type from MonadPrompt: forall x. (p a -> (a -> x) -> x) -> (r -> x) -> x) The main innovation in prompt seems to be the 't' argument, which according to the package's readme, can be used for short-circuiting evaluation and nondeterminism (branching computations). As I explained in my previous email, the 'p' argument in MonadPrompt is meant to describe the API of a monadic DSL. It appears that the two ideas could be combined in a single type, forall x. (p a -> (t a -> x) -> x) -> (t r -> x) -> x) or forall m. Monad m => (p a -> m (t a)) -> m (t r) or, using the free monad defined by the functor data PromptF a b x = forall a. PromptF (t a -> x) (p a) Perhaps this is worth investigating further. Cheers, Bertram