    {-# LANGUAGE ConstraintKinds #-}
    {-# LANGUAGE DataKinds #-}

    {-# LANGUAGE FlexibleContexts #-}
    {-# LANGUAGE FlexibleInstances #-}

    {-# LANGUAGE MultiParamTypeClasses #-}
    {-# LANGUAGE PolyKinds #-}
    {-# LANGUAGE RankNTypes #-}
    {-# LANGUAGE TypeFamilies #-}
    {-# LANGUAGE TypeOperators #-}
    {-# LANGUAGE UndecidableInstances #-}

    {-

    Copyright Dennis Gosnell (c) 2017-2018

    All rights reserved.

    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions are met:

        * Redistributions of source code must retain the above copyright
          notice, this list of conditions and the following disclaimer.

        * Redistributions in binary form must reproduce the above
          copyright notice, this list of conditions and the following
          disclaimer in the documentation and/or other materials provided
          with the distribution.

        * Neither the name of Author name here nor the names of other
          contributors may be used to endorse or promote products derived
          from this software without specific prior written permission.

    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
    A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
    OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
    SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
    LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
    DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
    THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
    (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
    OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

    -}

    -- | Type-level code for implementing and using 'UVerb'.  Heavily inspired by
    -- [world-peace](https://github.com/cdepillabout/world-peace).
    module Servant.API.UVerb.Union
    ( IsMember
    , Unique
    , Union
    , inject
    , eject
    , foldMapUnion
    , matchUnion
    )
    where

    import Data.Kind (Type)
    import Data.Proxy (Proxy)
    import Data.SOP.BasicFunctors (I, unI)
    import Data.SOP.Constraint
    import Data.SOP.NS
    import Data.Type.Bool (If)
    import Data.Type.Equality (type (==))
    import GHC.TypeLits

    type Union = NS I

    -- | Convenience function to apply a function to an unknown union element using a type class.
    -- All elements of the union must have instances in the type class, and the function is
    -- applied unconditionally.
    --
    -- See also: 'matchUnion'.
    foldMapUnion ::
      forall (c :: Type -> Constraint) (a :: Type) (as :: [Type]).
      All c as =>
      Proxy c ->
      (forall x. c x => x -> a) ->
      Union as ->
      a
    foldMapUnion :: forall (c :: Type -> Constraint) a (as :: [Type]).
    All c as =>
    Proxy c -> (forall x. c x => x -> a) -> Union as -> a
    foldMapUnion Proxy c
    proxy forall x. c x => x -> a
    go = Proxy c -> (forall a. c a => I a -> a) -> NS I as -> a
    forall {k} (c :: k -> Constraint)
           (proxy :: (k -> Constraint) -> Type) (f :: k -> Type) (xs :: [k])
           m.
    All c xs =>
    proxy c -> (forall (a :: k). c a => f a -> m) -> NS f xs -> m
    cfoldMap_NS Proxy c
    proxy (a -> a
    forall x. c x => x -> a
    go (a -> a) -> (I a -> a) -> I a -> a
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . I a -> a
    forall a. I a -> a
    unI)

    -- | Convenience function to extract a union element using 'cast', ie. return the value if the
    -- selected type happens to be the actual type of the union in this value, or 'Nothing'
    -- otherwise.
    --
    -- See also: 'foldMapUnion'.
    matchUnion :: forall (a :: Type) (as :: [Type]). (IsMember a as) => Union as -> Maybe a
    matchUnion :: forall a (as :: [Type]). IsMember a as => Union as -> Maybe a
    matchUnion = (I a -> a) -> Maybe (I a) -> Maybe a
    forall a b. (a -> b) -> Maybe a -> Maybe b
    forall (f :: Type -> Type) a b. Functor f => (a -> b) -> f a -> f b
    fmap I a -> a
    forall a. I a -> a
    unI (Maybe (I a) -> Maybe a)
    -> (Union as -> Maybe (I a)) -> Union as -> Maybe a
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Union as -> Maybe (I a)
    forall {k} (x :: k) (xs :: [k]) (f :: k -> Type).
    UElem x xs =>
    NS f xs -> Maybe (f x)
    forall (f :: Type -> Type). NS f as -> Maybe (f a)
    eject

    -- * Stuff stolen from 'Data.WorldPeace" but for generics-sop

    -- (this could to go sop-core, except it's probably too specialized to the servant use-case.)

    type IsMember (a :: u) (as :: [u]) = (Unique as, CheckElemIsMember a as, UElem a as)

    class UElem x xs where
      inject :: f x -> NS f xs
      eject :: NS f xs -> Maybe (f x)

    instance {-# OVERLAPPING #-} UElem x (x ': xs) where
      inject :: forall (f :: a -> Type). f x -> NS f (x : xs)
    inject = f x -> NS f (x : xs)
    forall {k} (a :: k -> Type) (x :: k) (xs :: [k]).
    a x -> NS a (x : xs)
    Z
      eject :: forall (f :: a -> Type). NS f (x : xs) -> Maybe (f x)
    eject (Z f x
    x) = f x -> Maybe (f x)
    forall a. a -> Maybe a
    Just f x
    f x
    x
      eject NS f (x : xs)
    _ = Maybe (f x)
    forall a. Maybe a
    Nothing

    instance {-# OVERLAPPING #-} UElem x xs => UElem x (x' ': xs) where
      inject :: forall (f :: a -> Type). f x -> NS f (x' : xs)
    inject = NS f xs -> NS f (x' : xs)
    forall {k} (a :: k -> Type) (xs :: [k]) (x :: k).
    NS a xs -> NS a (x : xs)
    S (NS f xs -> NS f (x' : xs))
    -> (f x -> NS f xs) -> f x -> NS f (x' : xs)
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . f x -> NS f xs
    forall {k} (x :: k) (xs :: [k]) (f :: k -> Type).
    UElem x xs =>
    f x -> NS f xs
    forall (f :: a -> Type). f x -> NS f xs
    inject
      eject :: forall (f :: a -> Type). NS f (x' : xs) -> Maybe (f x)
    eject (Z f x
    _) = Maybe (f x)
    forall a. Maybe a
    Nothing
      eject (S NS f xs
    ns) = NS f xs -> Maybe (f x)
    forall {k} (x :: k) (xs :: [k]) (f :: k -> Type).
    UElem x xs =>
    NS f xs -> Maybe (f x)
    forall (f :: a -> Type). NS f xs -> Maybe (f x)
    eject NS f xs
    ns

    -- | Check whether @a@ is in given type-level list.
    -- This will throw a nice error if the element is not in the list.
    type family CheckElemIsMember (a :: k) (as :: [k]) :: Constraint where
        CheckElemIsMember a as =
          If (Elem a as) (() :: Constraint) (TypeError (NoElementError a as))

    type NoElementError (r :: k) (rs :: [k]) =
              'Text "Expected one of:"
        ':$$: 'Text "    " ':<>: 'ShowType rs
        ':$$: 'Text "But got:"
        ':$$: 'Text "    " ':<>: 'ShowType r

    type DuplicateElementError (rs :: [k]) =
              'Text "Duplicate element in list:"
        ':$$: 'Text "    " ':<>: 'ShowType rs

    type family Elem (x :: k) (xs :: [k]) :: Bool where
      Elem x (x ': _) = 'True
      Elem x (_ ': xs) = Elem x xs
      Elem _ '[] = 'False

    -- | Check whether all values in a type-level list are distinct.
    -- This will throw a nice error if there are any duplicate elements in the list.
    type family Unique xs :: Constraint where
      Unique xs = If (Nubbed xs == 'True) (() :: Constraint) (TypeError (DuplicateElementError xs))

    type family Nubbed xs :: Bool where
      Nubbed '[] = 'True
      Nubbed (x ': xs) = If (Elem x xs) 'False (Nubbed xs)

    _testNubbed :: ( ( Nubbed '[Bool, Int, Int] ~ 'False
                     , Nubbed '[Int, Int, Bool] ~ 'False
                     , Nubbed '[Int, Bool] ~ 'True
                     )
                   => a) -> a
    _testNubbed :: forall a.
    ((Nubbed '[Bool, Int, Int] ~ 'False,
      Nubbed '[Int, Int, Bool] ~ 'False, Nubbed '[Int, Bool] ~ 'True) =>
     a)
    -> a
    _testNubbed (Nubbed '[Bool, Int, Int] ~ 'False,
     Nubbed '[Int, Int, Bool] ~ 'False, Nubbed '[Int, Bool] ~ 'True) =>
    a
    a = a
    (Nubbed '[Bool, Int, Int] ~ 'False,
     Nubbed '[Int, Int, Bool] ~ 'False, Nubbed '[Int, Bool] ~ 'True) =>
    a
    a