Replace substitution function with a map

It makes using variables with queries more approachable, but some work
still has to be done.
- The type `Subs` should be renamed and moved out of `Schema`, together with
`AST.Core.Value` probably.
- Some kind of conversion should be possible from a user-defined input
type T to the Value. So the final HashMap should have a type like
`HashMap name a`, where a is an instance of a potential typeclass
InputType.

src/Language/GraphQL.hs

@@ -20,7 +20,7 @@ graphql :: MonadIO m
     => NonEmpty (Schema.Resolver m) -- ^ Resolvers.
     -> T.Text -- ^ Text representing a @GraphQL@ request document.
     -> m Aeson.Value -- ^ Response.
-graphql = flip graphqlSubs $ const Nothing
+graphql = flip graphqlSubs mempty
 
 -- | If the text parses correctly as a @GraphQL@ query the substitution is
 -- applied to the query and the query is then executed using to the given

src/Language/GraphQL/Execute/Transform.hs

@@ -11,7 +11,7 @@ module Language.GraphQL.Execute.Transform
 import Control.Arrow (first)
 import Control.Monad (foldM, unless)
 import Control.Monad.Trans.Class (lift)
-import Control.Monad.Trans.Reader (ReaderT, ask, runReaderT)
+import Control.Monad.Trans.Reader (ReaderT, asks, runReaderT)
 import Control.Monad.Trans.State (StateT, evalStateT, gets, modify)
 import Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as HashMap
@@ -155,9 +155,7 @@ argument :: Full.Argument -> TransformT Core.Argument
 argument (Full.Argument n v) = Core.Argument n <$> value v
 
 value :: Full.Value -> TransformT Core.Value
-value (Full.Variable n) = do
-    substitute' <- lift ask
-    lift . lift $ substitute' n
+value (Full.Variable name) = lift (asks $ HashMap.lookup name) >>= lift . lift
 value (Full.Int i) = pure $ Core.Int i
 value (Full.Float f) = pure $ Core.Float f
 value (Full.String x) = pure $ Core.String x

src/Language/GraphQL/Schema.hs

@@ -44,8 +44,9 @@ data Resolver m = Resolver
     Text -- ^ Name
     (Field -> CollectErrsT m Aeson.Object) -- ^ Resolver
 
--- | Variable substitution function.
-type Subs = Name -> Maybe Value
+-- | Contains variables for the query. The key of the map is a variable name,
+--   and the value is the variable value.
+type Subs = HashMap Name Value
 
 -- | Create a new 'Resolver' with the given 'Name' from the given 'Resolver's.
 object :: MonadIO m => Name -> ActionT m [Resolver m] -> Resolver m