{-# LANGUAGE CPP #-}
{-# LANGUAGE LambdaCase #-}
module Data.GraphQL.Schema
  ( Schema(..)
  , Resolver
  , Subs
  , object
  , objectA
  , scalar
  , scalarA
  , array
  , arrayA
  , enum
  , enumA
  , resolvers
  , fields
  -- * AST Reexports
  , Field
  , Argument(..)
  , Value(..)
  , StringValue(..)
  ) where

#if !MIN_VERSION_base(4,8,0)
import Control.Applicative (pure, (<|>))
import Data.Foldable (foldMap)
import Data.Traversable (traverse)
import Data.Monoid (Monoid(mempty,mappend))
#else
import Data.Monoid (Alt(Alt,getAlt))
#endif
import Control.Applicative (Alternative, empty)
import Data.Maybe (catMaybes)
import Data.Foldable (fold)

import qualified Data.Aeson as Aeson
import Data.HashMap.Strict (HashMap)
import qualified Data.HashMap.Strict as HashMap
import Data.Text (Text)
import qualified Data.Text as T (null)

import Data.GraphQL.AST

data Schema f = Schema [Resolver f]

type Resolver  f = Field -> f Aeson.Object

type Subs = Text -> Maybe Text

object :: Alternative f => Text -> [Resolver f] -> Resolver f
object name resolvs = objectA name $ \case
     [] -> resolvs
     _  -> empty

objectA
  :: Alternative f
  => Text -> ([Argument] -> [Resolver f]) -> Resolver f
objectA name f fld@(Field _ _ args _ sels) =
    withField name (resolvers (f args) $ fields sels) fld

scalar :: (Alternative f, Aeson.ToJSON a) => Text -> a -> Resolver f
scalar name s = scalarA name $ \case
    [] -> pure s
    _  -> empty

scalarA
  :: (Alternative f, Aeson.ToJSON a)
  => Text -> ([Argument] -> f a) -> Resolver f
scalarA name f fld@(Field _ _ args _ []) = withField name (f args) fld
scalarA _ _ _ = empty

array :: Alternative f => Text -> [[Resolver f]] -> Resolver f
array name resolvs = arrayA name $ \case
    [] -> resolvs
    _  -> empty

arrayA
  :: Alternative f
  => Text -> ([Argument] -> [[Resolver f]]) -> Resolver f
arrayA name f fld@(Field _ _ args _ sels) =
     withField name (traverse (flip resolvers $ fields sels) $ f args) fld

enum :: Alternative f => Text -> f [Text] -> Resolver f
enum name enums = enumA name $ \case
     [] -> enums
     _  -> empty

enumA :: Alternative f => Text -> ([Argument] -> f [Text]) -> Resolver f
enumA name f fld@(Field _ _ args _ []) = withField name (f args) fld
enumA _ _ _ = empty

withField
  :: (Alternative f, Aeson.ToJSON a)
  => Text -> f a -> Field -> f (HashMap Text Aeson.Value)
withField name f (Field alias name' _ _ _) =
     if name == name'
        then fmap (HashMap.singleton aliasOrName . Aeson.toJSON) f
        else empty
     where
       aliasOrName = if T.null alias then name' else alias

resolvers :: Alternative f => [Resolver f] -> [Field] -> f Aeson.Value
resolvers resolvs =
    fmap (Aeson.toJSON . fold)
  . traverse (\fld -> getAlt $ foldMap (Alt . ($ fld)) resolvs)

field :: Selection -> Maybe Field
field (SelectionField x) = Just x
field _ = Nothing

fields :: SelectionSet -> [Field]
fields = catMaybes . fmap field

#if !MIN_VERSION_base(4,8,0)
newtype Alt f a = Alt {getAlt :: f a}

instance Alternative f => Monoid (Alt f a) where
        mempty = Alt empty
        Alt x `mappend` Alt y = Alt $ x <|> y
#endif