    -- | Constructing singleton collections.

    module Agda.Utils.Singleton where

    import Data.Semigroup (Semigroup(..))
    import Data.Monoid (Endo(..))

    import Data.Hashable (Hashable)
    import Data.HashMap.Strict (HashMap)
    import qualified Data.HashMap.Strict as HashMap
    import Data.HashSet (HashSet)
    import qualified Data.HashSet as HashSet
    import Data.List.NonEmpty (NonEmpty(..))

    import Data.IntMap (IntMap)
    import qualified Data.IntMap as IntMap
    import Data.IntSet (IntSet)
    import qualified Data.IntSet as IntSet

    import Data.Map (Map)
    import qualified Data.Map as Map
    import Data.Sequence (Seq)
    import qualified Data.Sequence as Seq
    import Data.Set (Set)
    import qualified Data.Set as Set

    -- | A create-only possibly empty collection is a monoid with the possibility
    --   to inject elements.

    class (Semigroup coll, Monoid coll, Singleton el coll) => Collection el coll
        | coll -> el where
      fromList :: [el] -> coll
      fromList = [coll] -> coll
    forall a. Monoid a => [a] -> a
    mconcat ([coll] -> coll) -> ([el] -> [coll]) -> [el] -> coll
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . (el -> coll) -> [el] -> [coll]
    forall a b. (a -> b) -> [a] -> [b]
    map el -> coll
    forall el coll. Singleton el coll => el -> coll
    singleton

    instance Collection a       [a]           where fromList :: [a] -> [a]
    fromList = [a] -> [a]
    forall a. a -> a
    id
    instance Collection a       ([a] -> [a])  where fromList :: [a] -> [a] -> [a]
    fromList = [a] -> [a] -> [a]
    forall a. [a] -> [a] -> [a]
    (++)
    instance Collection a       (Endo [a])    where fromList :: [a] -> Endo [a]
    fromList = ([a] -> [a]) -> Endo [a]
    forall a. (a -> a) -> Endo a
    Endo (([a] -> [a]) -> Endo [a])
    -> ([a] -> [a] -> [a]) -> [a] -> Endo [a]
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . [a] -> [a] -> [a]
    forall el coll. Collection el coll => [el] -> coll
    fromList
    instance Collection a       (Seq a)       where fromList :: [a] -> Seq a
    fromList = [a] -> Seq a
    forall a. [a] -> Seq a
    Seq.fromList
    instance Collection Int     IntSet        where fromList :: [Int] -> IntSet
    fromList = [Int] -> IntSet
    IntSet.fromList
    instance Collection (Int,a) (IntMap a)    where fromList :: [(Int, a)] -> IntMap a
    fromList = [(Int, a)] -> IntMap a
    forall a. [(Int, a)] -> IntMap a
    IntMap.fromList

    instance Ord a =>
             Collection a       (Set a)       where fromList :: [a] -> Set a
    fromList = [a] -> Set a
    forall a. Ord a => [a] -> Set a
    Set.fromList

    instance Ord k =>
             Collection (k, a)  (Map k a)     where fromList :: [(k, a)] -> Map k a
    fromList = [(k, a)] -> Map k a
    forall k a. Ord k => [(k, a)] -> Map k a
    Map.fromList

    instance (Eq a, Hashable a) =>
             Collection a       (HashSet a)   where fromList :: [a] -> HashSet a
    fromList = [a] -> HashSet a
    forall a. (Eq a, Hashable a) => [a] -> HashSet a
    HashSet.fromList

    instance (Eq k, Hashable k) =>
             Collection (k, a)  (HashMap k a) where fromList :: [(k, a)] -> HashMap k a
    fromList = [(k, a)] -> HashMap k a
    forall k a. (Eq k, Hashable k) => [(k, a)] -> HashMap k a
    HashMap.fromList

    -- | Overloaded @singleton@ constructor for collections.

    class Singleton el coll | coll -> el where
      singleton :: el -> coll

    instance Singleton a   (Maybe a)   where singleton :: a -> Maybe a
    singleton = a -> Maybe a
    forall a. a -> Maybe a
    Just
    instance Singleton a   [a]         where singleton :: a -> [a]
    singleton = (a -> [a] -> [a]
    forall a. a -> [a] -> [a]
    :[])
    instance Singleton a  ([a] -> [a]) where singleton :: a -> [a] -> [a]
    singleton = (:)
    instance Singleton a   (Endo [a])  where singleton :: a -> Endo [a]
    singleton = ([a] -> [a]) -> Endo [a]
    forall a. (a -> a) -> Endo a
    Endo (([a] -> [a]) -> Endo [a]) -> (a -> [a] -> [a]) -> a -> Endo [a]
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . (:)
    instance Singleton a   (NonEmpty a)
                                       where singleton :: a -> NonEmpty a
    singleton = (a -> [a] -> NonEmpty a
    forall a. a -> [a] -> NonEmpty a
    :| [])
    instance Singleton a   (Seq a)     where singleton :: a -> Seq a
    singleton = a -> Seq a
    forall a. a -> Seq a
    Seq.singleton
    instance Singleton a   (Set a)     where singleton :: a -> Set a
    singleton = a -> Set a
    forall a. a -> Set a
    Set.singleton
    instance Singleton Int IntSet      where singleton :: Int -> IntSet
    singleton = Int -> IntSet
    IntSet.singleton

    instance Singleton (k  ,a) (Map  k a) where singleton :: (k, a) -> Map k a
    singleton = (k -> a -> Map k a) -> (k, a) -> Map k a
    forall a b c. (a -> b -> c) -> (a, b) -> c
    uncurry k -> a -> Map k a
    forall k a. k -> a -> Map k a
    Map.singleton
    instance Singleton (Int,a) (IntMap a) where singleton :: (Int, a) -> IntMap a
    singleton = (Int -> a -> IntMap a) -> (Int, a) -> IntMap a
    forall a b c. (a -> b -> c) -> (a, b) -> c
    uncurry Int -> a -> IntMap a
    forall a. Int -> a -> IntMap a
    IntMap.singleton

    instance Hashable a => Singleton a     (HashSet   a) where singleton :: a -> HashSet a
    singleton = a -> HashSet a
    forall a. Hashable a => a -> HashSet a
    HashSet.singleton
    instance Hashable k => Singleton (k,a) (HashMap k a) where singleton :: (k, a) -> HashMap k a
    singleton = (k -> a -> HashMap k a) -> (k, a) -> HashMap k a
    forall a b c. (a -> b -> c) -> (a, b) -> c
    uncurry k -> a -> HashMap k a
    forall k v. Hashable k => k -> v -> HashMap k v
    HashMap.singleton

    -- Testing newtype-deriving:

    -- newtype Wrap c = Wrap c
    --   deriving (Singleton k)  -- Succeeds

    -- Type family version:

    -- class Singleton c where
    --   type Elem c
    --   singleton :: Elem c -> c

    -- instance Singleton [a] where
    --   type Elem [a] = a
    --   singleton = (:[])

    -- instance Singleton (Maybe a) where
    --   type Elem (Maybe a) = a
    --   singleton = Just

    -- instance Singleton (Set a) where
    --   type Elem (Set a) = a
    --   singleton = Set.singleton

    -- instance Singleton (Map k a) where
    --   type Elem (Map k a) = (k,a)
    --   singleton = uncurry Map.singleton

    -- newtype Wrap a = Wrap a
    --   deriving (Singleton)  -- Fails