Define resolvers on type fields

Returning resolvers from other resolvers isn't supported anymore. Since
we have a type system now, we define the resolvers in the object type
fields and pass an object with the previous result to them.

src/Language/GraphQL/AST/Core.hs

@@ -1,37 +1,15 @@
 -- | This is the AST meant to be executed.
 module Language.GraphQL.AST.Core
-    ( Alias
-    , Arguments(..)
-    , Directive(..)
-    , Field(..)
-    , Fragment(..)
+    ( Arguments(..)
     , Name
-    , Operation(..)
-    , Selection(..)
-    , TypeCondition
     ) where
 
 import Data.HashMap.Strict (HashMap)
-import Data.Sequence (Seq)
-import Data.Text (Text)
-import Language.GraphQL.AST (Alias, Name, TypeCondition)
-import qualified Language.GraphQL.Type.In as In
-
--- | GraphQL has 3 operation types: queries, mutations and subscribtions.
---
--- Currently only queries and mutations are supported.
-data Operation
-    = Query (Maybe Text) (Seq Selection)
-    | Mutation (Maybe Text) (Seq Selection)
-    deriving (Eq, Show)
-
--- | Single GraphQL field.
-data Field
-    = Field (Maybe Alias) Name Arguments (Seq Selection)
-    deriving (Eq, Show)
+import Language.GraphQL.AST (Name)
+import Language.GraphQL.Type.Definition
 
 -- | Argument list.
-newtype Arguments = Arguments (HashMap Name In.Value)
+newtype Arguments = Arguments (HashMap Name Value)
     deriving (Eq, Show)
 
 instance Semigroup Arguments where
@@ -40,17 +18,3 @@ instance Semigroup Arguments where
 instance Monoid Arguments where
     mempty = Arguments mempty
 
--- | Directive.
-data Directive = Directive Name Arguments
-    deriving (Eq, Show)
-
--- | Represents fragments and inline fragments.
-data Fragment
-    = Fragment TypeCondition (Seq Selection)
-    deriving (Eq, Show)
-
--- | Single selection element.
-data Selection
-    = SelectionFragment Fragment
-    | SelectionField Field
-    deriving (Eq, Show)