{-# LANGUAGE DataKinds                #-}
{-# LANGUAGE GADTs                    #-}
{-# LANGUAGE PolyKinds                #-}
{-# LANGUAGE RankNTypes               #-}
{-# LANGUAGE StandaloneKindSignatures #-}
{-# LANGUAGE TypeFamilies             #-}
{-# LANGUAGE TypeOperators            #-}

module Polysemy.Several
  ( HList (..),
    TypeMap,
    Append,
    runSeveral,
  )
where

import           Data.Kind (Type)
import           Polysemy  (Sem)

------------------------------------------------------------------------------

-- | A list capable of storing values of different types. Creating an HList
-- looks like.
--
-- > 1 ::: "test" ::: True ::: HNil
infixr 5 :::

type HList :: [Type] -> Type
data HList a where
  HNil :: HList '[]
  (:::) :: a -> HList (b :: [Type]) -> HList (a ': b)

------------------------------------------------------------------------------

-- | A map function over type level lists. For example, the following two
-- lines are equivalent:
--
-- > TypeMap Reader [Int, String, False]
-- > [Reader Int, Reader String, Reader Bool]
type TypeMap :: (a -> b) -> [a] -> [b]
type family TypeMap (f :: a -> b) (xs :: [a]) where
  TypeMap _ '[] = '[]
  TypeMap f (x ': xs) = f x ': TypeMap f xs

------------------------------------------------------------------------------

-- | Type-level append.
--
-- > Append [Int, String] [Bool]
-- > [Int, String, Bool]
type Append :: [t] -> [t] -> [t]
type family Append (a :: [t]) (b :: [t]) where
  Append '[] b = b
  Append (a ': as) b = a ': Append as b

------------------------------------------------------------------------------

-- | A helper function for building new runners which accept HLists intsead of
-- individual elements. If you would normally write
--
-- > f 5 . f "Text" . f True
--
-- then this function can turn that into
--
-- > runSeveral f (True ::: "Text" ::: 5 ::: HNil)
--
-- For example, a runReaders function could be implemented as:
--
-- > runReaders :: HList t -> Sem (TypeConcat (TypeMap Reader t) r) a -> Sem r a
-- > runReaders = runSeveral runReader
--
-- @since 0.1.0.0
runSeveral ::
  (forall r' k x. k -> Sem (e k ': r') x -> Sem r' x) ->
  HList t ->
  Sem (Append (TypeMap e t) r) a ->
  Sem r a
runSeveral :: forall (e :: * -> Effect) (t :: [*]) (r :: [Effect]) a.
(forall (r' :: [Effect]) k x. k -> Sem (e k : r') x -> Sem r' x)
-> HList t -> Sem (Append (TypeMap e t) r) a -> Sem r a
runSeveral forall (r' :: [Effect]) k x. k -> Sem (e k : r') x -> Sem r' x
f (a
a ::: HList b
as) = forall (e :: * -> Effect) (t :: [*]) (r :: [Effect]) a.
(forall (r' :: [Effect]) k x. k -> Sem (e k : r') x -> Sem r' x)
-> HList t -> Sem (Append (TypeMap e t) r) a -> Sem r a
runSeveral forall (r' :: [Effect]) k x. k -> Sem (e k : r') x -> Sem r' x
f HList b
as forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall (r' :: [Effect]) k x. k -> Sem (e k : r') x -> Sem r' x
f a
a
runSeveral forall (r' :: [Effect]) k x. k -> Sem (e k : r') x -> Sem r' x
_ HList t
HNil       = forall a. a -> a
id