2 files changed, 16 insertions, 2 deletions
diff --git a/src/Language/GraphQL/Validate.hs b/src/Language/GraphQL/Validate.hs
index 6df97a1..5768615 100644
--- a/src/Language/GraphQL/Validate.hs
+++ b/src/Language/GraphQL/Validate.hs
@@ -5,6 +5,7 @@
 {-# LANGUAGE ExplicitForAll #-}
 {-# LANGUAGE LambdaCase #-}
 
+-- | GraphQL validator.
 module Language.GraphQL.Validate
     ( Error(..)
     , Path(..)
@@ -29,17 +30,24 @@ data Context m = Context
 
 type ValidateT m = Reader (Context m) (Seq Error)
 
+-- | If an error can be associated to a particular field in the GraphQL result,
+-- it must contain an entry with the key path that details the path of the
+-- response field which experienced the error. This allows clients to identify
+-- whether a null result is intentional or caused by a runtime error.
 data Path
-    = Segment Text
-    | Index Int
+    = Segment Text -- ^ Field name.
+    | Index Int -- ^ List index if a field returned a list.
     deriving (Eq, Show)
 
+-- | Validation error.
 data Error = Error
     { message :: String
     , locations :: [Location]
     , path :: [Path]
     } deriving (Eq, Show)
 
+-- | Validates a document and returns a list of found errors. If the returned
+-- list is empty, the document is valid.
 document :: forall m. Schema m -> [Rule] -> Document -> Seq Error
 document schema' rules' document' =
     runReader (foldrM go Seq.empty document') context
diff --git a/src/Language/GraphQL/Validate/Rules.hs b/src/Language/GraphQL/Validate/Rules.hs
index bc754c2..a3314e7 100644
--- a/src/Language/GraphQL/Validate/Rules.hs
+++ b/src/Language/GraphQL/Validate/Rules.hs
@@ -2,6 +2,7 @@
    v. 2.0. If a copy of the MPL was not distributed with this file, You can
    obtain one at https://mozilla.org/MPL/2.0/. -}
 
+-- | This module contains default rules defined in the GraphQL specification.
 module Language.GraphQL.Validate.Rules
     ( Rule(..)
     , executableDefinitionsRule
@@ -10,14 +11,19 @@ module Language.GraphQL.Validate.Rules
 
 import Language.GraphQL.AST.Document
 
+-- | 'Rule' assigns a function to each AST node that can be validated. If the
+-- validation fails, the function should return an error message, or 'Nothing'
+-- otherwise.
 newtype Rule
     = DefinitionRule (Definition -> Maybe String)
 
+-- | Default reules given in the specification.
 specifiedRules :: [Rule]
 specifiedRules =
     [ executableDefinitionsRule
     ]
 
+-- | Definition must be OperationDefinition or FragmentDefinition.
 executableDefinitionsRule :: Rule
 executableDefinitionsRule = DefinitionRule go
   where