{-# LANGUAGE CPP #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE NoMonomorphismRestriction #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE RecursiveDo #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
#ifdef USE_REFLEX_OPTIMIZER
{-# OPTIONS_GHC -fplugin=Reflex.Optimizer #-}
#endif
-- |
-- Module:
--   Reflex.Dynamic
-- Description:
--   This module contains various functions for working with 'Dynamic' values.
--   'Dynamic' and its primitives have been moved to the 'Reflex' class.
module Reflex.Dynamic
  ( -- * Basics
    Dynamic -- Abstract so we can preserve the law that the current value is always equal to the most recent update
  , current
  , updated
  , holdDyn
  , mapDynM
  , forDynM
  , constDyn
  , count
  , toggle
  , switchDyn
  , switchPromptlyDyn
  , tagPromptlyDyn
  , attachPromptlyDyn
  , attachPromptlyDynWith
  , attachPromptlyDynWithMaybe
  , maybeDyn
  , eitherDyn
  , factorDyn
  , scanDyn
  , scanDynMaybe
  , holdUniqDyn
  , holdUniqDynBy
  , improvingMaybe
  , foldDyn
  , foldDynM
  , foldDynMaybe
  , foldDynMaybeM
  , joinDynThroughMap
  , traceDyn
  , traceDynWith
  , splitDynPure
  , distributeMapOverDynPure
  , distributeDMapOverDynPure
  , distributeListOverDynPure
  , Demux
  , demux
  , demuxed
    -- * Miscellaneous
    -- Things that probably aren't very useful:
  , HList (..)
  , FHList (..)
  , collectDynPure
  , RebuildSortedHList (..)
  , IsHList (..)
  , AllAreFunctors (..)
  , HListPtr (..)
  , distributeFHListOverDynPure
    -- * Unsafe
  , unsafeDynamic
  ) where

import Data.Functor.Compose
import Data.Functor.Misc
import Reflex.Class

import Control.Monad
import Control.Monad.Fix
import Control.Monad.Identity
import Data.Align
import Data.Dependent.Map (DMap)
import qualified Data.Dependent.Map as DMap
import Data.Dependent.Sum (DSum (..))
import Data.GADT.Compare ((:~:) (..), GCompare (..), GEq (..), GOrdering (..))
import Data.Map (Map)
import Data.Maybe
import Data.Monoid ((<>))
import Data.These

import Debug.Trace

-- | Map a sampling function over a 'Dynamic'.
mapDynM :: forall t m a b. (Reflex t, MonadHold t m) => (forall m'. MonadSample t m' => a -> m' b) -> Dynamic t a -> m (Dynamic t b)
mapDynM f d = buildDynamic (f =<< sample (current d)) $ pushAlways f (updated d)

-- | Flipped version of 'mapDynM'
forDynM :: forall t m a b. (Reflex t, MonadHold t m) => Dynamic t a -> (forall m'. MonadSample t m' => a -> m' b) -> m (Dynamic t b)
forDynM d f = mapDynM f d

-- | Create a new 'Dynamic' that only signals changes if the values actually
-- changed.
holdUniqDyn :: (Reflex t, MonadHold t m, MonadFix m, Eq a) => Dynamic t a -> m (Dynamic t a)
holdUniqDyn = holdUniqDynBy (==)

-- | Create a new 'Dynamic' that changes only when the underlying 'Dynamic'
-- changes and the given function returns 'False' when given both the old and
-- the new values.
holdUniqDynBy :: (Reflex t, MonadHold t m, MonadFix m) => (a -> a -> Bool) -> Dynamic t a -> m (Dynamic t a)
holdUniqDynBy eq = scanDynMaybe id (\new old -> if new `eq` old then Nothing else Just new)

-- | @/Dynamic Maybe/@ that can only update from @/Nothing/@ to @/Just/@ or @/Just/@ to @/Just/@ (i.e., cannot revert to @/Nothing/@)
improvingMaybe :: (Reflex t, MonadHold t m, MonadFix m) => Dynamic t (Maybe a) -> m (Dynamic t (Maybe a))
improvingMaybe = scanDynMaybe id (\new _ -> if isJust new then Just new else Nothing)

-- | Create a 'Dynamic' that accumulates values from another 'Dynamic'.  This
-- function does not force its input 'Dynamic' until the output 'Dynamic' is
-- forced.
scanDyn :: (Reflex t, MonadHold t m, MonadFix m) => (a -> b) -> (a -> b -> b) -> Dynamic t a -> m (Dynamic t b)
scanDyn z f = scanDynMaybe z (\a b -> Just $ f a b)

-- | Like 'scanDyn', but the the accumulator function may decline to update the
-- result 'Dynamic''s value.
scanDynMaybe :: (Reflex t, MonadHold t m, MonadFix m) => (a -> b) -> (a -> b -> Maybe b) -> Dynamic t a -> m (Dynamic t b)
scanDynMaybe z f d = do
  rec d' <- buildDynamic (z <$> sample (current d)) $ flip push (updated d) $ \a -> do
        b <- sample $ current d'
        return $ f a b
  return d'

-- | Create a 'Dynamic' using the initial value and change it each time the
-- 'Event' occurs using a folding function on the previous value and the value
-- of the 'Event'.
foldDyn :: (Reflex t, MonadHold t m, MonadFix m) => (a -> b -> b) -> b -> Event t a -> m (Dynamic t b)
foldDyn = accumDyn . flip

-- | Like 'foldDyn', but the combining function is a 'PushM' action, so it
-- can 'sample' existing 'Behaviors' and 'hold' new ones.
foldDynM :: (Reflex t, MonadHold t m, MonadFix m) => (a -> b -> PushM t b) -> b -> Event t a -> m (Dynamic t b)
foldDynM = accumMDyn . flip

-- | Create a 'Dynamic' using the provided initial value and change it each time
-- the provided 'Event' occurs, using a function to combine the old value with
-- the 'Event''s value.  If the function returns 'Nothing', the value is not
-- changed; this is distinct from returning 'Just' the old value, since the
-- 'Dynamic''s 'updated' 'Event' will fire in the 'Just' case, and will not fire
-- in the 'Nothing' case.
foldDynMaybe :: (Reflex t, MonadHold t m, MonadFix m) => (a -> b -> Maybe b) -> b -> Event t a -> m (Dynamic t b)
foldDynMaybe = accumMaybeDyn . flip

-- | Like 'foldDynMaybe', but the combining function is a 'PushM' action, so it
-- can 'sample' existing 'Behaviors' and 'hold' new ones.
foldDynMaybeM :: (Reflex t, MonadHold t m, MonadFix m) => (a -> b -> PushM t (Maybe b)) -> b -> Event t a -> m (Dynamic t b)
foldDynMaybeM = accumMaybeMDyn . flip

-- | Create a new 'Dynamic' that counts the occurrences of the 'Event'.
count :: (Reflex t, MonadHold t m, MonadFix m, Num b) => Event t a -> m (Dynamic t b)
count e = holdDyn 0 =<< zipListWithEvent const (iterate (+1) 1) e

-- | Create a new 'Dynamic' using the initial value that flips its
-- value every time the 'Event' occurs.
toggle :: (Reflex t, MonadHold t m, MonadFix m) => Bool -> Event t a -> m (Dynamic t Bool)
toggle = foldDyn (const not)

-- | Switches to the new 'Event' whenever it receives one. Only the old event is
-- considered the moment a new one is switched in; the output event will fire at
-- that moment if only if the old event does.
--
-- Prefer this to 'switchPromptlyDyn' where possible. The lack of doing double
-- work when the outer and (new) inner fires means this imposes fewer "timing
-- requirements" and thus is far more easy to use without introducing fresh
-- failure cases. 'switchDyn' is also more performant.
switchDyn :: forall t a. Reflex t => Dynamic t (Event t a) -> Event t a
switchDyn d = switch (current d)

-- | Switches to the new 'Event' whenever it receives one.  Switching occurs
-- __before__ the inner 'Event' fires - so if the 'Dynamic' changes and both the
-- old and new inner Events fire simultaneously, the output will fire with the
-- value of the __new__ 'Event'.
--
-- Prefer 'switchDyn' to this where possible. The timing requirements that
-- switching before imposes are likely to bring down your app unless you are
-- very careful. 'switchDyn' is also more performant.
switchPromptlyDyn :: forall t a. Reflex t => Dynamic t (Event t a) -> Event t a
switchPromptlyDyn de =
  let eLag = switch $ current de
      eCoincidences = coincidence $ updated de
  in leftmost [eCoincidences, eLag]

-- | Split a 'Dynamic' pair into a pair of 'Dynamic's
splitDynPure :: Reflex t => Dynamic t (a, b) -> (Dynamic t a, Dynamic t b)
splitDynPure d = (fmap fst d, fmap snd d)

-- | Convert a 'Map' with 'Dynamic' elements into a 'Dynamic' of a 'Map' with
-- non-'Dynamic' elements.
distributeMapOverDynPure :: (Reflex t, Ord k) => Map k (Dynamic t v) -> Dynamic t (Map k v)
distributeMapOverDynPure = fmap dmapToMap . distributeDMapOverDynPure . mapWithFunctorToDMap

-- | Convert a list with 'Dynamic' elements into a 'Dynamic' of a list with
-- non-'Dynamic' elements, preserving the order of the elements.
distributeListOverDynPure :: Reflex t => [Dynamic t v] -> Dynamic t [v]
distributeListOverDynPure =
  fmap (map fromDSum . DMap.toAscList) .
  distributeDMapOverDynPure .
  DMap.fromDistinctAscList .
  zipWith toDSum [0..]
  where
    toDSum :: Int -> Dynamic t a -> DSum (Const2 Int a) (Dynamic t)
    toDSum k v = Const2 k :=> v
    fromDSum :: DSum (Const2 Int a) Identity -> a
    fromDSum (Const2 _ :=> Identity v) = v

--TODO: Generalize this to functors other than Maps
-- | Combine a 'Dynamic' of a 'Map' of 'Dynamic's into a 'Dynamic'
-- with the current values of the 'Dynamic's in a map.
joinDynThroughMap :: forall t k a. (Reflex t, Ord k) => Dynamic t (Map k (Dynamic t a)) -> Dynamic t (Map k a)
joinDynThroughMap = (distributeMapOverDynPure =<<)

-- | Print the value of the 'Dynamic' when it is first read and on each
-- subsequent change that is observed (as 'traceEvent'), prefixed with the
-- provided string. This should /only/ be used for debugging.
--
-- Note: Just like Debug.Trace.trace, the value will only be shown if something
-- else in the system is depending on it.
traceDyn :: (Reflex t, Show a) => String -> Dynamic t a -> Dynamic t a
traceDyn s = traceDynWith $ \x -> s <> ": " <> show x

-- | Print the result of applying the provided function to the value
-- of the 'Dynamic' when it is first read and on each subsequent change
-- that is observed (as 'traceEvent'). This should /only/ be used for
-- debugging.
--
-- Note: Just like Debug.Trace.trace, the value will only be shown if something
-- else in the system is depending on it.
traceDynWith :: Reflex t => (a -> String) -> Dynamic t a -> Dynamic t a
traceDynWith f d =
  let e' = traceEventWith f $ updated d
      getV0 = do
        x <- sample $ current d
        trace (f x) $ return x
  in unsafeBuildDynamic getV0 e'

-- | Replace the value of the 'Event' with the current value of the 'Dynamic'
-- each time the 'Event' occurs.
--
-- Note: @/tagPromptlyDyn d e/@ differs from @/tag (current d) e/@ in the case that @/e/@ is firing
-- at the same time that @/d/@ is changing.  With @/tagPromptlyDyn d e/@, the __new__ value of @/d/@
-- will replace the value of @/e/@, whereas with @/tag (current d) e/@, the __old__ value
-- will be used, since the 'Behavior' won't be updated until the end of the frame.
-- Additionally, this means that the output 'Event' may not be used to directly change
-- the input 'Dynamic', because that would mean its value depends on itself.  __When creating__
-- __cyclic data flows, generally @/tag (current d) e/@ is preferred.__
tagPromptlyDyn :: Reflex t => Dynamic t a -> Event t b -> Event t a
tagPromptlyDyn = attachPromptlyDynWith const

-- | Attach the current value of the 'Dynamic' to the value of the
-- 'Event' each time it occurs.
--
-- Note: @/attachPromptlyDyn d/@ is not the same as @/attach (current d)/@.  See 'tagPromptlyDyn' for details.
attachPromptlyDyn :: Reflex t => Dynamic t a -> Event t b -> Event t (a, b)
attachPromptlyDyn = attachPromptlyDynWith (,)

-- | Combine the current value of the 'Dynamic' with the value of the
-- 'Event' each time it occurs.
--
-- Note: @/attachPromptlyDynWith f d/@ is not the same as @/attachWith f (current d)/@.  See 'tagPromptlyDyn' for details.
attachPromptlyDynWith :: Reflex t => (a -> b -> c) -> Dynamic t a -> Event t b -> Event t c
attachPromptlyDynWith f = attachPromptlyDynWithMaybe $ \a b -> Just $ f a b

-- | Create a new 'Event' by combining the value at each occurrence with the
-- current value of the 'Dynamic' value and possibly filtering if the combining
-- function returns 'Nothing'.
--
-- Note: @/attachPromptlyDynWithMaybe f d/@ is not the same as @/attachWithMaybe f (current d)/@.  See 'tagPromptlyDyn' for details.
attachPromptlyDynWithMaybe :: Reflex t => (a -> b -> Maybe c) -> Dynamic t a -> Event t b -> Event t c
attachPromptlyDynWithMaybe f d e =
  let e' = attach (current d) e
  in fforMaybe (align e' $ updated d) $ \case
       This (a, b) -> f a b -- Only the tagging event is firing, so use that
       These (_, b) a -> f a b -- Both events are firing, so use the newer value
       That _ -> Nothing -- The tagging event isn't firing, so don't fire

-- | Factor a @/Dynamic t (Maybe a)/@ into a @/Dynamic t (Maybe (Dynamic t a))/@,
-- such that the outer 'Dynamic' is updated only when the "Maybe"'s constructor
-- chages from 'Nothing' to 'Just' or vice-versa.  Whenever the constructor
-- becomes 'Just', an inner 'Dynamic' will be provided, whose value will track
-- the 'a' inside the 'Just'; when the constructor becomes 'Nothing', the
-- existing inner 'Dynamic' will become constant, and will not change when the
-- outer constructor changes back to 'Nothing'.
maybeDyn :: forall t a m. (Reflex t, MonadFix m, MonadHold t m) => Dynamic t (Maybe a) -> m (Dynamic t (Maybe (Dynamic t a)))
maybeDyn = fmap (fmap unpack) . eitherDyn . fmap pack
  where pack = \case
          Nothing -> Left ()
          Just a -> Right a
        unpack = \case
          Left _ -> Nothing
          Right a -> Just a

eitherDyn :: forall t a b m. (Reflex t, MonadFix m, MonadHold t m) => Dynamic t (Either a b) -> m (Dynamic t (Either (Dynamic t a) (Dynamic t b)))
eitherDyn = fmap (fmap unpack) . factorDyn . fmap eitherToDSum
  where unpack :: DSum (EitherTag a b) (Compose (Dynamic t) Identity) -> Either (Dynamic t a) (Dynamic t b)
        unpack = \case
          LeftTag :=> Compose a -> Left $ coerceDynamic a
          RightTag :=> Compose b -> Right $ coerceDynamic b

factorDyn :: forall t m k v. (Reflex t, MonadHold t m, GEq k)
          => Dynamic t (DSum k v) -> m (Dynamic t (DSum k (Compose (Dynamic t) v)))
factorDyn d = buildDynamic (sample (current d) >>= holdKey) update  where
  update :: Event t (DSum k (Compose (Dynamic t) v))
  update = flip push (updated d) $ \(newKey :=> newVal) -> do
     (oldKey :=> _) <- sample (current d)
     case newKey `geq` oldKey of
      Just Refl -> return Nothing
      Nothing -> Just <$> holdKey (newKey :=> newVal)

  holdKey (k :=> v) = do
    inner' <- filterEventKey k (updated d)
    inner <- holdDyn v inner'
    return $ k :=> Compose inner
--------------------------------------------------------------------------------
-- Demux
--------------------------------------------------------------------------------

-- | Represents a time changing value together with an 'EventSelector' that can
-- efficiently detect when the underlying 'Dynamic' has a particular value.
-- This is useful for representing data like the current selection of a long
-- list.
--
-- Semantically,
--
-- > demuxed (demux d) k === fmap (== k) d
--
-- However, when getDemuxed is used multiple times, the complexity is only
-- /O(log(n))/, rather than /O(n)/ for fmap.
data Demux t k = Demux { demuxValue :: Behavior t k
                       , demuxSelector :: EventSelector t (Const2 k Bool)
                       }

-- | Demultiplex an input value to a 'Demux' with many outputs.  At any given
-- time, whichever output is indicated by the given 'Dynamic' will be 'True'.
demux :: (Reflex t, Ord k) => Dynamic t k -> Demux t k
demux k = Demux (current k)
                (fan $ attachWith (\k0 k1 -> if k0 == k1
                                                then DMap.empty
                                                else DMap.fromList [Const2 k0 :=> Identity False,
                                                                    Const2 k1 :=> Identity True])
                                  (current k) (updated k))

-- | Select a particular output of the 'Demux'; this is equivalent to (but much
-- faster than) mapping over the original 'Dynamic' and checking whether it is
-- equal to the given key.
demuxed :: (Reflex t, Eq k) => Demux t k -> k -> Dynamic t Bool
demuxed d k =
  let e = select (demuxSelector d) (Const2 k)
  in unsafeBuildDynamic (fmap (==k) $ sample $ demuxValue d) e

--------------------------------------------------------------------------------
-- collectDyn
--------------------------------------------------------------------------------

--TODO: This whole section is badly in need of cleanup

-- | A heterogeneous list whose type and length are fixed statically.  This is
-- reproduced from the 'HList' package due to integration issues, and because
-- very little other functionality from that library is needed.
data HList (l::[*]) where
  HNil  :: HList '[]
  HCons :: e -> HList l -> HList (e ': l)

infixr 2 `HCons`

type family HRevApp (l1 :: [k]) (l2 :: [k]) :: [k]
type instance HRevApp '[] l = l
type instance HRevApp (e ': l) l' = HRevApp l (e ': l')

hRevApp :: HList l1 -> HList l2 -> HList (HRevApp l1 l2)
hRevApp HNil l = l
hRevApp (HCons x l) l' = hRevApp l (HCons x l')

hReverse :: HList l -> HList (HRevApp l '[])
hReverse l = hRevApp l HNil

hBuild :: (HBuild' '[] r) => r
hBuild =  hBuild' HNil

class HBuild' l r where
    hBuild' :: HList l -> r

instance (l' ~ HRevApp l '[])
      => HBuild' l (HList l') where
  hBuild' = hReverse

instance HBuild' (a ': l) r
      => HBuild' l (a->r) where
  hBuild' l x = hBuild' (HCons x l)

-- | Like 'HList', but with a functor wrapping each element.
data FHList f l where
  FHNil :: FHList f '[]
  FHCons :: f e -> FHList f l -> FHList f (e ': l)

instance GEq (HListPtr l) where
  HHeadPtr `geq` HHeadPtr = Just Refl
  HHeadPtr `geq` HTailPtr _ = Nothing
  HTailPtr _ `geq` HHeadPtr = Nothing
  HTailPtr a `geq` HTailPtr b = a `geq` b

instance GCompare (HListPtr l) where -- Warning: This ordering can't change, dmapTo*HList will break
  HHeadPtr `gcompare` HHeadPtr = GEQ
  HHeadPtr `gcompare` HTailPtr _ = GLT
  HTailPtr _ `gcompare` HHeadPtr = GGT
  HTailPtr a `gcompare` HTailPtr b = a `gcompare` b

-- | A typed index into a typed heterogeneous list.
data HListPtr l a where
  HHeadPtr :: HListPtr (h ': t) h
  HTailPtr :: HListPtr t a -> HListPtr (h ': t) a

deriving instance Eq (HListPtr l a)
deriving instance Ord (HListPtr l a)

fhlistToDMap :: forall (f :: * -> *) l. FHList f l -> DMap (HListPtr l) f
fhlistToDMap = DMap.fromList . go
  where go :: forall l'. FHList f l' -> [DSum (HListPtr l') f]
        go = \case
          FHNil -> []
          FHCons h t -> (HHeadPtr :=> h) : map (\(p :=> v) -> HTailPtr p :=> v) (go t)

-- | This class allows 'HList's and 'FHlist's to be built from regular lists;
-- they must be contiguous and sorted.
class RebuildSortedHList l where
  rebuildSortedFHList :: [DSum (HListPtr l) f] -> FHList f l
  rebuildSortedHList :: [DSum (HListPtr l) Identity] -> HList l

instance RebuildSortedHList '[] where
  rebuildSortedFHList l = case l of
    [] -> FHNil
    _ : _ -> error "rebuildSortedFHList{'[]}: empty list expected"
  rebuildSortedHList l = case l of
    [] -> HNil
    _ : _ -> error "rebuildSortedHList{'[]}: empty list expected"

instance RebuildSortedHList t => RebuildSortedHList (h ': t) where
  rebuildSortedFHList l = case l of
    ((HHeadPtr :=> h) : t) -> FHCons h . rebuildSortedFHList . map (\(HTailPtr p :=> v) -> p :=> v) $ t
    _ -> error "rebuildSortedFHList{h':t}: non-empty list with HHeadPtr expected"
  rebuildSortedHList l = case l of
    ((HHeadPtr :=> Identity h) : t) -> HCons h . rebuildSortedHList . map (\(HTailPtr p :=> v) -> p :=> v) $ t
    _ -> error "rebuildSortedHList{h':t}: non-empty list with HHeadPtr expected"

dmapToHList :: forall l. RebuildSortedHList l => DMap (HListPtr l) Identity -> HList l
dmapToHList = rebuildSortedHList . DMap.toList

-- | Collect a hetereogeneous list whose elements are all 'Dynamic's into a
-- single 'Dynamic' whose value represents the current values of all of the
-- input 'Dynamic's.
distributeFHListOverDynPure :: (Reflex t, RebuildSortedHList l) => FHList (Dynamic t) l -> Dynamic t (HList l)
distributeFHListOverDynPure l = fmap dmapToHList $ distributeDMapOverDynPure $ fhlistToDMap l

-- | Indicates that all elements in a type-level list are applications of the
-- same functor.
class AllAreFunctors (f :: a -> *) (l :: [a]) where
  type FunctorList f l :: [*]
  toFHList :: HList (FunctorList f l) -> FHList f l
  fromFHList :: FHList f l -> HList (FunctorList f l)

instance AllAreFunctors f '[] where
  type FunctorList f '[] = '[]
  toFHList l = case l of
    HNil -> FHNil
#if !defined(__GLASGOW_HASKELL__) || __GLASGOW_HASKELL__ < 800
    _ -> error "toFHList: impossible" -- Otherwise, GHC complains of a non-exhaustive pattern match; see https://ghc.haskell.org/trac/ghc/ticket/4139
#endif
  fromFHList FHNil = HNil

instance AllAreFunctors f t => AllAreFunctors f (h ': t) where
  type FunctorList f (h ': t) = f h ': FunctorList f t
  toFHList l = case l of
    a `HCons` b -> a `FHCons` toFHList b
#if !defined(__GLASGOW_HASKELL__) || __GLASGOW_HASKELL__ < 800
    _ -> error "toFHList: impossible" -- Otherwise, GHC complains of a non-exhaustive pattern match; see https://ghc.haskell.org/trac/ghc/ticket/4139
#endif
  fromFHList (a `FHCons` b) = a `HCons` fromFHList b

-- | Convert a datastructure whose constituent parts are all 'Dynamic's into a
-- single 'Dynamic' whose value represents all the current values of the input's
-- constituent 'Dynamic's.
collectDynPure :: ( RebuildSortedHList (HListElems b)
                  , IsHList a, IsHList b
                  , AllAreFunctors (Dynamic t) (HListElems b)
                  , Reflex t
                  , HListElems a ~ FunctorList (Dynamic t) (HListElems b)
                  ) => a -> Dynamic t b
collectDynPure ds = fmap fromHList $ distributeFHListOverDynPure $ toFHList $ toHList ds

-- | Poor man's 'Generic's for product types only.
class IsHList a where
  type HListElems a :: [*]
  toHList :: a -> HList (HListElems a)
  fromHList :: HList (HListElems a) -> a

instance IsHList (a, b) where
  type HListElems (a, b) = [a, b]
  toHList (a, b) = hBuild a b
  fromHList l = case l of
    a `HCons` b `HCons` HNil -> (a, b)
#if !defined(__GLASGOW_HASKELL__) || __GLASGOW_HASKELL__ < 800
    _ -> error "fromHList: impossible" -- Otherwise, GHC complains of a non-exhaustive pattern match; see https://ghc.haskell.org/trac/ghc/ticket/4139
#endif

instance IsHList (a, b, c, d) where
  type HListElems (a, b, c, d) = [a, b, c, d]
  toHList (a, b, c, d) = hBuild a b c d
  fromHList l = case l of
    a `HCons` b `HCons` c `HCons` d `HCons` HNil -> (a, b, c, d)
#if !defined(__GLASGOW_HASKELL__) || __GLASGOW_HASKELL__ < 800
    _ -> error "fromHList: impossible" -- Otherwise, GHC complains of a non-exhaustive pattern match; see https://ghc.haskell.org/trac/ghc/ticket/4139
#endif

instance IsHList (a, b, c, d, e, f) where
  type HListElems (a, b, c, d, e, f) = [a, b, c, d, e, f]
  toHList (a, b, c, d, e, f) = hBuild a b c d e f
  fromHList l = case l of
    a `HCons` b `HCons` c `HCons` d `HCons` e `HCons` f `HCons` HNil -> (a, b, c, d, e, f)
#if !defined(__GLASGOW_HASKELL__) || __GLASGOW_HASKELL__ < 800
    _ -> error "fromHList: impossible" -- Otherwise, GHC complains of a non-exhaustive pattern match; see https://ghc.haskell.org/trac/ghc/ticket/4139
#endif