{-# LANGUAGE GADTs #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
module Control.Algebra.Free
(
-- * Classes
AlgebraType0
, AlgebraType
, Proof (..)
, FreeAlgebra1 (..)
-- * Combinators
, wrapFree
, foldFree1
, unFoldNatFree
, hoistFree1
, hoistFreeH
, joinFree1
, bindFree1
, assocFree1
, iterFree1
, cataFree1
-- * Day convolution
, DayF (..)
, dayToAp
, apToDay
-- * Various classes (higher algebra types)
, MonadList (..)
, MonadMaybe (..)
) where
import Control.Applicative (Alternative)
import Control.Applicative.Free (Ap)
import qualified Control.Applicative.Free as Ap
import qualified Control.Applicative.Free.Fast as Fast
import qualified Control.Applicative.Free.Final as Final
import Control.Alternative.Free (Alt (..))
import qualified Control.Alternative.Free as Alt
import Control.Monad (foldM, join)
import Control.Monad.Except (ExceptT (..), MonadError (..))
import Control.Monad.Free (Free)
import qualified Control.Monad.Free as Free
import qualified Control.Monad.Free.Church as Church
import Control.Monad.List (ListT (..))
import Control.Monad.Reader (MonadReader (..), ReaderT (..))
import Control.Monad.RWS.Class (MonadRWS)
import Control.Monad.RWS.Lazy as L (RWST (..))
import Control.Monad.RWS.Strict as S (RWST (..))
import Control.Monad.State.Class (MonadState (..))
import qualified Control.Monad.State.Lazy as L (StateT (..))
import qualified Control.Monad.State.Strict as S (StateT (..))
import Control.Monad.Trans (lift)
import Control.Monad.Trans.Maybe (MaybeT (..))
import Control.Monad.Writer.Class (MonadWriter (..))
import qualified Control.Monad.Writer.Lazy as L (WriterT (..))
import qualified Control.Monad.Writer.Strict as S (WriterT (..))
import Data.Constraint (Dict (..))
import Data.Kind (Type)
import Data.Fix (Fix, cataM)
import Data.Functor.Coyoneda (Coyoneda (..), liftCoyoneda)
import Data.Functor.Day (Day (..))
import qualified Data.Functor.Day as Day
import Data.Functor.Identity (Identity (..))
import Data.Algebra.Free (AlgebraType, AlgebraType0, Proof (..))
-- |
-- Higher kinded version of @'FreeAlgebra'@. Instances includes free functors,
-- free applicative functors, free monads, state monads etc.
--
-- A lawful instance should guarantee that @'foldNatFree'@ is an isomorphism
-- with inverses @'unFoldNatFree'@.
--
-- This guaranties that @m@ is a left adjoint functor from the category of
-- types of kind @Type -> Type@ which satisfy @'AlgebraType0' m@ constraint, to the
-- category of types of kind @Type -> Type@ which satisfy the @'AlgebraType' m@
-- constraint. This functor is left adjoin to the forgetful functor (which is
-- well defined if the laws on @'AlgebraType0'@ family are satisfied. This in
-- turn guarantees that @m@ composed with this forgetful functor is a monad.
-- In result we get the monadic combinators: @'liftFree'@ (@'return'@ of
-- this monad) and @'bindFree1'@ (its @'bind'@) and @'joinFree1'@ - its
-- @'join'@ operator.
class FreeAlgebra1 (m :: (Type -> Type) -> Type -> Type) where
-- | Natural transformation that embeds generators into @m@.
liftFree :: AlgebraType0 m f => f a -> m f a
-- | The freeness property.
foldNatFree
:: forall (d :: Type -> Type) f a .
( AlgebraType m d
, AlgebraType0 m f
)
=> (forall x. f x -> d x)
-- ^ natural transformation which embeds generators of @m@ into @d@
-> (m f a -> d a)
-- ^ a homomorphism from @m f@ to @d@
-- |
-- A proof that @'AlgebraType' m (m f)@ holds for all @AlgebraType0 f => f@.
-- Together with @hoistFree1@ this proves that @FreeAlgebra m => m@ is
-- a functor from the full subcategory of types of kind @Type -> Type@
-- which satisfy @'AlgebraType0' m f@ to ones that satisfy @'AlgebraType'
-- m f@.
proof1 :: forall f. AlgebraType0 m f => Proof (AlgebraType m (m f)) (m f)
-- |
-- A proof that the forgetful functor from the full subcategory of types of
-- kind @Type -> Type@ satisfying @'AlgebraType' m f@ constraint to types
-- satisfying @'AlgebraType0' m f@ is well defined.
forget1 :: forall f. AlgebraType m f => Proof (AlgebraType0 m f) (m f)
-- |
-- Anything that carries @'FreeAlgebra1'@ constraint is also an instance of
-- @'Control.Monad.Free.Class.MonadFree'@, but not vice versa. You can use
-- @'wrap'@ to define a @'Contorl.Monad.Free.Class.MonadFree'@ instance.
-- @'ContT'@ is an example of a monad which does have an @'FreeAlgebra1'@
-- instance, but has an @'MonadFree'@ instance.
--
-- The @'Monad'@ constrain will be satisfied for many monads through the
-- @'AlgebraType m'@ constraint.
wrapFree
:: ( FreeAlgebra1 m
, AlgebraType0 m f
, Monad (m f)
)
=> f (m f a)
-> m f a
wrapFree = join . liftFree
-- |
-- @'FreeAlgebra1' m@ implies that @m f@ is a foldable.
--
-- @
-- 'foldFree1' . 'liftFree' == 'id' :: f a -> f a
-- @
--
-- @foldFree1@ is the
-- [unit](https://ncatlab.org/nlab/show/unit+of+an+adjunction) of the
-- adjunction imposed by @FreeAlgebra1@ constraint.
--
-- It can be specialized to:
--
-- * @'Data.Functor.Coyoneda.lowerCoyoneda' :: 'Functor' f => 'Coyoneda' f a -> f a@
-- * @'Control.Applicative.Free.retractAp' :: 'Applicative' f => 'Ap' f a -> f a@
-- * @'Control.Monad.Free.foldFree' :: 'Monad' m => (forall x. f x -> m x) -> 'Free' f a -> m a@
foldFree1 :: forall m f a .
( FreeAlgebra1 m
, AlgebraType m f
)
=> m f a
-> f a
foldFree1 = case forget1 @m @f of
Proof Dict -> foldNatFree id
-- |
-- @'unFoldNatFree'@ is an inverse of @'foldNatFree'@
--
-- prop> unFoldNatFree id = ruturnFree1
--
-- Note that @'unFoldNatFree' id@ is the
-- [unit](https://ncatlab.org/nlab/show/unit+of+an+adjunction) of the
-- adjunction imposed by the @'FreeAlgebra1'@ constraint.
unFoldNatFree
:: ( FreeAlgebra1 m
, AlgebraType0 m f
)
=> (forall x . m f x -> d x)
-> f a -> d a
unFoldNatFree nat = nat . liftFree
-- |
-- This is a functor instance for @m@ when considered as an endofuctor of some
-- subcategory of @Type -> Type@ (e.g. endofunctors of _Hask_).
--
-- It can be specialized to:
--
-- * @'Control.Applicative.Free.hoistAp' :: (forall a. f a -> g a) -> 'Ap' f b -> 'Ap' g b @
-- * @'Control.Monad.Free.hoistFree' :: 'Functor' g => (forall a. f a -> g a) -> 'Free' f b -> 'Free' g b@
hoistFree1 :: forall m f g a .
( FreeAlgebra1 m
, AlgebraType0 m g
, AlgebraType0 m f
)
=> (forall x. f x -> g x) -- ^ a natural transformation @f ~> g@
-> m f a
-> m g a
hoistFree1 nat = case proof1 @m @g of
Proof Dict -> foldNatFree (liftFree . nat)
-- |
-- @
-- 'hoistFreeH' . 'hoistFreeH' = 'hoistFreeH'
-- @
--
-- and when @'FreeAlgebra1' m ~ 'FreeAlgebra1' n@:
--
-- @
-- 'hoistFreeH' = 'id'
-- @
hoistFreeH :: forall m n f a .
( FreeAlgebra1 m
, FreeAlgebra1 n
, AlgebraType0 m f
, AlgebraType0 n f
, AlgebraType m (n f)
)
=> m f a
-> n f a
hoistFreeH = foldNatFree liftFree
-- |
-- @'joinFree1'@ makes @m@ a monad in some subcatgory of types of kind @Type -> Type@
-- (usually the endo-functor category of @Hask@). It is just a specialization
-- of @'foldFree1'@.
joinFree1 :: forall m f a .
( FreeAlgebra1 m
, AlgebraType0 m f
)
=> m (m f) a
-> m f a
joinFree1 = case proof1 @m @f of
Proof Dict -> case forget1 @m @(m f) of
Proof Dict -> foldFree1
-- |
-- Bind operator for the @'joinFree1'@ monad, this is just @'foldNatFree'@ in
-- disguise.
bindFree1 :: forall m f g a .
( FreeAlgebra1 m
, AlgebraType0 m g
, AlgebraType0 m f
)
=> m f a
-> (forall x . f x -> m g x) -- ^ natural transformation @f ~> m g@
-> m g a
bindFree1 mfa nat = case proof1 @m @g of
Proof Dict -> foldNatFree nat mfa
assocFree1 :: forall m f a .
( FreeAlgebra1 m
, AlgebraType m f
, Functor (m (m f))
)
=> m f (m f a)
-> m (m f) (f a)
assocFree1 = case forget1 @m @f of
Proof Dict -> case proof1 @m @f of
Proof Dict -> case forget1 @m @(m f) of
Proof Dict -> case proof1 @m @(m f) of
Proof Dict -> case forget1 @m @(m (m f)) of
Proof Dict -> fmap g <$> foldNatFree f
where
f :: forall x .
( AlgebraType0 m f
, AlgebraType0 m (m f)
)
=> f x
-> m (m f) x
f = hoistFree1 liftFree . liftFree
g :: m f a -> f a
g = foldFree1
-- |
-- @'Fix' (m f)@ is the initial /algebra/ of type @'AlgebraType' m@ and
-- @'AlgebraType0' f@ (whenever it /exists/).
cataFree1 :: forall m f a .
( FreeAlgebra1 m
, AlgebraType m f
, Monad f
, Traversable (m f)
)
=> Fix (m f)
-> f a
cataFree1 = cataM foldFree1
-- |
-- Specialization of @'foldNatFree' \@_ \@'Identity'@; it will further specialize to:
--
-- * @\\_ -> 'runIdentity' . 'Data.Functor.Coyoneda.lowerCoyoneda'@
-- * @'Control.Applicative.Free.iterAp' :: 'Functor' g => (g a -> a) -> 'Ap' g a -> a@
-- * @'Control.Monad.Free.iter' :: 'Functor' f => (f a -> a) -> 'Free' f a -> a@
iterFree1 :: forall m f a .
( FreeAlgebra1 m
, AlgebraType0 m f
, AlgebraType m Identity
)
=> (forall x . f x -> x)
-> m f a
-> a
iterFree1 f = runIdentity . foldNatFree @_ @Identity (Identity . f)
-- Instances
-- |
-- Algebras of the same type as @'Coyoneda'@ are all functors.
type instance AlgebraType0 Coyoneda g = ()
type instance AlgebraType Coyoneda g = Functor g
instance FreeAlgebra1 Coyoneda where
liftFree = liftCoyoneda
foldNatFree nat (Coyoneda ba fx) = ba <$> nat fx
proof1 = Proof Dict
forget1 = Proof Dict
-- |
-- Algebras of the same type as @'Ap'@ are the applicative functors.
type instance AlgebraType0 Ap g = Functor g
type instance AlgebraType Ap g = Applicative g
-- |
-- @'Ap'@ is a free in the class of applicative functors, over any functor
-- (@'Ap' f@ is applicative whenever @f@ is a functor)
instance FreeAlgebra1 Ap where
liftFree = Ap.liftAp
foldNatFree = Ap.runAp
proof1 = Proof Dict
forget1 = Proof Dict
type instance AlgebraType0 Fast.Ap g = Functor g
type instance AlgebraType Fast.Ap g = Applicative g
instance FreeAlgebra1 Fast.Ap where
liftFree = Fast.liftAp
foldNatFree = Fast.runAp
proof1 = Proof Dict
forget1 = Proof Dict
type instance AlgebraType0 Final.Ap g = Functor g
type instance AlgebraType Final.Ap g = Applicative g
instance FreeAlgebra1 Final.Ap where
liftFree = Final.liftAp
foldNatFree = Final.runAp
proof1 = Proof Dict
forget1 = Proof Dict
-- |
-- @'Day' f f@ newtype wrapper. It is isomorphic with @'Ap' f@ for applicative
-- functors @f@ via @'dayToAp'@ (and @'apToDay'@).
newtype DayF f a = DayF { runDayF :: Day f f a}
deriving (Functor, Applicative)
dayToAp :: Applicative f => Day f f a -> Ap f a
dayToAp = hoistFreeH . DayF
apToDay :: Applicative f => Ap f a -> Day f f a
apToDay = runDayF . hoistFreeH
-- |
-- Algebras of the same type as @'DayF'@ are all the applicative functors.
type instance AlgebraType0 DayF g = Applicative g
type instance AlgebraType DayF g = Applicative g
-- |
-- @'DayF'@, as @'Ap'@ is a free applicative functor, but over applicative functors
-- (@'DayF' f@ is applicative if @f@ is an applicative functor).
instance FreeAlgebra1 DayF where
liftFree fa = DayF $ Day fa fa const
foldNatFree nat (DayF day)
= Day.dap . Day.trans2 nat . Day.trans1 nat $ day
proof1 = Proof Dict
forget1 = Proof Dict
-- |
-- Algebras of the same type as @'Free'@ monad is the class of all monads.
type instance AlgebraType0 Free f = Functor f
type instance AlgebraType Free m = Monad m
-- |
-- @'Free'@ monad is free in the class of monad over the class of functors.
instance FreeAlgebra1 Free where
liftFree = Free.liftF
foldNatFree = Free.foldFree
proof1 = Proof Dict
forget1 = Proof Dict
type instance AlgebraType0 Church.F f = Functor f
type instance AlgebraType Church.F m = Monad m
instance FreeAlgebra1 Church.F where
liftFree = Church.liftF
foldNatFree = Church.foldF
proof1 = Proof Dict
forget1 = Proof Dict
type instance AlgebraType0 Alt f = Functor f
type instance AlgebraType Alt m = Alternative m
instance FreeAlgebra1 Alt where
liftFree = Alt.liftAlt
foldNatFree = Alt.runAlt
proof1 = Proof Dict
forget1 = Proof Dict
-- |
-- Algebras of the same type as @'L.StateT'@ monad is the class of all state
-- monads.
type instance AlgebraType0 (L.StateT s) m = Monad m
type instance AlgebraType (L.StateT s) m = ( MonadState s m )
-- |
-- Lazy @'L.StateT'@ monad transformer is a free algebra in the class of monads
-- which satisfy the @'MonadState'@ constraint. Note that this instance
-- captures that @'L.StateT' s@ is a monad transformer:
--
-- @
-- 'liftFree' = 'lift'
-- @
--
-- This is also true for all the other monad transformers.
instance FreeAlgebra1 (L.StateT s) where
liftFree = lift
foldNatFree nat ma = do
(a, s) <- get >>= nat . L.runStateT ma
put s
return a
proof1 = Proof Dict
forget1 = Proof Dict
-- |
-- Algebras of the same type as @'S.StateT'@ monad is the class of all state
-- monads.
type instance AlgebraType0 (S.StateT s) m = Monad m
type instance AlgebraType (S.StateT s) m = ( MonadState s m )
-- |
-- Strict @'S.StateT'@ monad transformer is also a free algebra, thus @'hoistFreeH'@
-- is an isomorphism between the strict and lazy versions.
instance FreeAlgebra1 (S.StateT s) where
liftFree :: Monad m => m a -> S.StateT s m a
liftFree = lift
foldNatFree nat ma = do
(a, s) <- get >>= nat . S.runStateT ma
put s
return a
proof1 = Proof Dict
forget1 = Proof Dict
-- |
-- Algebras of the same type as @'L.WriterT'@ monad is the class of all writer
-- monads.
type instance AlgebraType0 (L.WriterT w) m = ( Monad m, Monoid w )
type instance AlgebraType (L.WriterT w) m = ( MonadWriter w m )
-- |
-- Lazy @'L.WriterT'@ is free for algebras of type @'MonadWriter'@.
instance FreeAlgebra1 (L.WriterT w) where
liftFree = lift
foldNatFree nat (L.WriterT m) = fst <$> nat m
proof1 = Proof Dict
forget1 = Proof Dict
-- |
-- Algebras of the same type as @'S.WriterT'@ monad is the class of all writer
-- monads.
type instance AlgebraType0 (S.WriterT w) m = ( Monad m, Monoid w )
type instance AlgebraType (S.WriterT w) m = ( MonadWriter w m )
-- |
-- Strict @'S.WriterT'@ monad transformer is a free algebra among all
-- @'MonadWriter'@s.
instance FreeAlgebra1 (S.WriterT w) where
liftFree = lift
foldNatFree nat (S.WriterT m) = fst <$> nat m
proof1 = Proof Dict
forget1 = Proof Dict
-- |
-- Algebras of the same type as @'L.ReaderT'@ monad is the class of all reader
-- monads.
type instance AlgebraType0 (ReaderT r) m = ( Monad m )
type instance AlgebraType (ReaderT r) m = ( MonadReader r m )
-- |
-- @'ReaderT'@ is a free monad in the class of all @'MonadReader'@ monads.
instance FreeAlgebra1 (ReaderT r) where
liftFree = lift
foldNatFree nat (ReaderT g) =
ask >>= nat . g
proof1 = Proof Dict
forget1 = Proof Dict
-- |
-- Algebras of the same type as @'S.ReaderT'@ monad is the class of all reader
-- monads.
type instance AlgebraType0 (ExceptT e) m = ( Monad m )
type instance AlgebraType (ExceptT e) m = ( MonadError e m )
-- |
-- @'ExceptT' e@ is a free algebra among all @'MonadError' e@ monads.
instance FreeAlgebra1 (ExceptT e) where
liftFree = lift
foldNatFree nat (ExceptT m) = do
ea <- nat m
case ea of
Left e -> throwError e
Right a -> return a
proof1 = Proof Dict
forget1 = Proof Dict
type instance AlgebraType0 (L.RWST r w s) m = ( Monad m, Monoid w )
type instance AlgebraType (L.RWST r w s) m = MonadRWS r w s m
instance FreeAlgebra1 (L.RWST r w s) where
liftFree = lift
foldNatFree nat (L.RWST fn) = do
r <- ask
s <- get
(a, s', w) <- nat $ fn r s
put s'
tell w
return a
proof1 = Proof Dict
forget1 = Proof Dict
type instance AlgebraType0 (S.RWST r w s) m = ( Monad m, Monoid w )
type instance AlgebraType (S.RWST r w s) m = MonadRWS r w s m
instance FreeAlgebra1 (S.RWST r w s) where
liftFree = lift
foldNatFree nat (S.RWST fn) = do
r <- ask
s <- get
(a, s', w) <- nat $ fn r s
put s'
tell w
return a
proof1 = Proof Dict
forget1 = Proof Dict
-- |
-- Algebra type for @'ListT'@ monad transformer.
class Monad m => MonadList m where
mempty1 :: m a
mappend1 :: m a -> m a -> m a
mappend1_ :: MonadList m => a -> a -> m a
mappend1_ a b = return a `mappend1` return b
instance Monad m => MonadList (ListT m) where
mempty1 = ListT (return [])
mappend1 (ListT ma) (ListT mb) = ListT $ mappend <$> ma <*> mb
type instance AlgebraType0 ListT f = ( Monad f )
type instance AlgebraType ListT m = ( MonadList m )
instance FreeAlgebra1 ListT where
liftFree = lift
foldNatFree nat (ListT mas) = do
as <- nat mas
empty <- mempty1
a <- foldM (\x y -> x `mappend1_` y) empty as
return a
proof1 = Proof Dict
forget1 = Proof Dict
-- $monadContT
--
-- @'ContT' r m@ is not functorial in @m@, so there is no chance it can admit
-- an instance of @'FreeAlgebra1'@
-- |
-- A higher version @'Data.Algebra.Pointed'@ class.
--
-- With @'QuantifiedConstraints'@ this class will be redundant.
class MonadMaybe m where
point :: forall a. m a
instance Monad m => MonadMaybe (MaybeT m) where
point = MaybeT (return Nothing)
type instance AlgebraType0 MaybeT m = ( Monad m )
type instance AlgebraType MaybeT m = ( Monad m, MonadMaybe m )
instance FreeAlgebra1 MaybeT where
liftFree = lift
foldNatFree nat (MaybeT mma) =
nat mma >>= \ma -> case ma of
Nothing -> point
Just a -> return a
proof1 = Proof Dict
forget1 = Proof Dict