Add OrderedMap prototype

src/Language/GraphQL/Execute/OrderedMap.hs

@@ -0,0 +1,64 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
+{-# LANGUAGE ExplicitForAll #-}
+module Language.GraphQL.Execute.OrderedMap
+    ( OrderedMap
+    , singleton
+    ) where
+
+import Data.HashMap.Strict (HashMap)
+import qualified Data.HashMap.Strict as HashMap
+import Data.Text (Text)
+import Data.Vector (Vector)
+import qualified Data.Vector as Vector
+
+data OrderedMap v = OrderedMap (Vector Text) (HashMap Text v)
+
+instance Functor OrderedMap where
+    fmap f (OrderedMap vector hashMap) = OrderedMap vector $ fmap f hashMap
+
+instance Foldable OrderedMap where
+    foldr f = foldrWithKey $ const f
+    null (OrderedMap vector _) = Vector.null vector
+
+instance Show v => Show (OrderedMap v) where
+    showsPrec precedence map' = showParen (precedence > 10)
+        $ showString "fromList " . shows (toList map')
+
+instance Semigroup (OrderedMap v) where
+    (<>) = foldrWithKey go
+      where
+        go key value accumulator@(OrderedMap vector hashMap)
+            | Nothing <- HashMap.lookup key hashMap
+                = OrderedMap (Vector.snoc vector key)
+                $ HashMap.insert key value hashMap
+            | otherwise = accumulator
+
+instance Monoid (OrderedMap v) where
+    mempty = OrderedMap mempty mempty
+
+-- * Construction
+
+-- | Constructs a map with a single element.
+singleton :: forall v. Text -> v -> OrderedMap v
+singleton key value = OrderedMap (Vector.singleton key)
+    $ HashMap.singleton key value
+
+-- * Folds
+
+-- | Reduces this map by applying a binary operator to all elements, using the
+-- given starting value.
+foldrWithKey :: forall v a. (Text -> v -> a -> a) -> a -> OrderedMap v -> a
+foldrWithKey f initial (OrderedMap vector hashMap) = foldr go initial vector
+  where
+    go key accumulator = f key (hashMap HashMap.! key) accumulator
+
+-- * Lists
+
+-- | Converts this map to the list of key-value pairs.
+toList :: forall v. OrderedMap v -> [(Text, v)]
+toList = foldrWithKey f []
+  where
+    f key value accumulator = (key, value) : accumulator