Define schema AST.

Large parts of the schema aren't exported publically. They will be made
public during writing the parser.

Fixes #6.

src/Language/GraphQL/AST/DirectiveLocation.hs

@@ -1,14 +1,20 @@
+-- | Various parts of a GraphQL document can be annotated with directives. 
+--   This module describes locations in a document where directives can appear.
 module Language.GraphQL.AST.DirectiveLocation
-    ( DirectiveLocation
-    , ExecutableDirectiveLocation
-    , TypeSystemDirectiveLocation
+    ( DirectiveLocation(..)
+    , ExecutableDirectiveLocation(..)
+    , TypeSystemDirectiveLocation(..)
     ) where
 
+-- | All directives can be splitted in two groups: directives used to annotate
+--   various parts of executable definitions and the ones used in the schema
+--   definition.
 data DirectiveLocation
     = ExecutableDirectiveLocation ExecutableDirectiveLocation
     | TypeSystemDirectiveLocation TypeSystemDirectiveLocation
     deriving (Eq, Show)
 
+-- | Where directives can appear in an executable definition, like a query.
 data ExecutableDirectiveLocation
     = Query
     | Mutation
@@ -19,6 +25,7 @@ data ExecutableDirectiveLocation
     | InlineFragment
     deriving (Eq, Show)
 
+-- | Where directives can appear in a type system definition.
 data TypeSystemDirectiveLocation
     = Schema
     | Scalar