Introduce `Node` data type to simplify `OperationDefinition`

Thanks to @swolchok for the suggestion.

Data/GraphQL/AST.hs

@@ -16,12 +16,12 @@ data Definition = DefinitionOperation OperationDefinition
                 | DefinitionType      TypeDefinition
                   deriving (Eq,Show)
 
-data OperationDefinition =
-    Query        Name [VariableDefinition] [Directive] SelectionSet
-  | Mutation     Name [VariableDefinition] [Directive] SelectionSet
-  -- Not official yet
-  -- -- | Subscription Name [VariableDefinition] [Directive] SelectionSet
-    deriving (Eq,Show)
+data OperationDefinition = Query    Node
+                         | Mutation Node
+                           deriving (Eq,Show)
+
+data Node = Node Name [VariableDefinition] [Directive] SelectionSet
+            deriving (Eq,Show)
 
 data VariableDefinition = VariableDefinition Variable Type (Maybe DefaultValue)
                           deriving (Eq,Show)

Data/GraphQL/Parser.hs

@@ -45,7 +45,6 @@ name = tok $ append <$> takeWhile1 isA_z
     -- `isAlpha` handles many more Unicode Chars
     isA_z =  inClass $ '_' : ['A'..'Z'] ++ ['a'..'z']
 
-
 -- * Document
 
 document :: Parser Document
@@ -54,7 +53,8 @@ document = whiteSpace
     -- Try SelectionSet when no definition
     <|> (Document . pure
                   . DefinitionOperation
-                  . Query mempty empty empty
+                  . Query
+                  . Node mempty empty empty
                 <$> selectionSet)
     <?> "document error!"
 
@@ -66,14 +66,15 @@ definition = DefinitionOperation <$> operationDefinition
 
 operationDefinition :: Parser OperationDefinition
 operationDefinition =
-        op Query "query"
-    <|> op Mutation "mutation"
+        Query    <$ tok "query"    <*> node
+    <|> Mutation <$ tok "mutation" <*> node
     <?> "operationDefinition error!"
-  where
-    op f n = f <$ tok n <*> tok name
-                        <*> optempty variableDefinitions
-                        <*> optempty directives
-                        <*> selectionSet
+
+node :: Parser Node
+node = Node <$> name
+            <*> optempty variableDefinitions
+            <*> optempty directives
+            <*> selectionSet
 
 variableDefinitions :: Parser [VariableDefinition]
 variableDefinitions = parens (many1 variableDefinition)