interruptible/src/Control/Monad/Trans/Interruptible/Class.hs

{-# LANGUAGE TypeFamilies #-}

module Control.Monad.Trans.Interruptible.Class (
  Interruptible(..),
  -- * Resumers for stacks of interruptibles
  resume2,
  resume3,
  resume4,
  resume5
  )where

import Control.Monad.Trans.Class
import Control.Monad.Trans.State
import Control.Monad.Trans.Either

{- |
Interruptible monad transformers.

A monad transformer can be interrupted if it returns its
final context from its type creator, and if it is possible
to hoist this context again into the monad at the begining
of its execution.

For example, @StateT@ can be interrupted because
@runStateT@ returns its final state, and because its state
can be set at the type creation. Error can not be hoisted,
thus is can not be interrupted.

Interruptible transformers can be stacked so that their
execution is resumed by composition of their @resume@
functions, and their data by the composition of their data
constructors at the inverse order. That is, in the stack:

> (Monad m, Interruptible i, Interruptible j) => i j m

Both i and j can be resumed by the function @resume . resume@,
and given @initI :: a -> RSt i a@ and @initJ :: a -> RSt j a@,
the total context is given by @initJ . initI@.

The context data constructors vary with each Interruptible,
as well as its signature.
-}
class MonadTrans t => Interruptible t where
  -- | Context data of the transformer
  type RSt t a :: *
  -- | Resumes the execution of an interruptible transformer
  resume :: Monad m => (a -> t m b) -> RSt t a -> m (RSt t b)

instance Interruptible (EitherT e) where
  -- | The context of @EitherT e a@ is @Either e a@.
  type RSt (EitherT e) a = Either e a
  resume f st = runEitherT (hoistEither st >>= f)

instance Interruptible (StateT st) where
  -- | The context of @StateT st a@ is @(a, st)@
  type RSt (StateT st) a = (a, st)
  resume f (a, st) = runStateT (f a) st

resume2 :: (Monad m, Interruptible t, Monad (t m), Interruptible u) =>
           (a -> u (t m) b) -> RSt t (RSt u a) -> m (RSt t (RSt u b))
resume2 = resume.resume

resume3 :: (Monad m, Interruptible t0, Monad (t0 m), Interruptible t1,
            Monad (t1 (t0 m)), Interruptible t2) =>
           (a -> t2 (t1 (t0 m)) b) -> RSt t0 (RSt t1 (RSt t2 a)) ->
           m (RSt t0 (RSt t1 (RSt t2 b)))
resume3 = resume2.resume

resume4 :: (Monad m, Interruptible t0, Interruptible t1, Interruptible t2,
            Interruptible t3, Monad (t0 m), Monad (t1 (t0 m)), Monad (t2 (t1 (t0 m)))) =>
           (a -> t3 (t2 (t1 (t0 m))) b) -> RSt t0 (RSt t1 (RSt t2 (RSt t3 a))) ->
           m (RSt t0 (RSt t1 (RSt t2 (RSt t3 b))))
resume4 = resume3.resume

resume5 :: (Monad m, Interruptible t0, Interruptible t1, Interruptible t2,
            Interruptible t3, Interruptible t4, Monad (t0 m), Monad (t1 (t0 m)),
            Monad (t2 (t1 (t0 m))), Monad (t3 (t2 (t1 (t0 m))))) =>
           (a -> t4 (t3 (t2 (t1 (t0 m)))) b) -> RSt t0 (RSt t1 (RSt t2 (RSt t3 (RSt t4 a)))) ->
           m (RSt t0 (RSt t1 (RSt t2 (RSt t3 (RSt t4 b)))))
resume5 = resume4.resume