diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0f7f370..ae76a2c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -28,6 +28,7 @@ and this project adheres to
   and event stream) resolvers to signalize an error. Other exceptions will
   escape.
 - `Test.Hspec.GraphQL` contains some test helpers.
+- `Validate` contains the validator and standard rules.
 
 ## Changed
 - `Type.Out.Resolver`: Interface fields don't have resolvers, object fields
diff --git a/graphql.cabal b/graphql.cabal
index eb514e0..6ac951a 100644
--- a/graphql.cabal
+++ b/graphql.cabal
@@ -4,10 +4,10 @@ cabal-version: 1.12
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: ba234bcfff46df053a3466359e32682c4592b88894911ecbe78bd00fa00929b5
+-- hash: 5fa34f11ab1242b72c75c3c19b31ba310c24716a0eb2d134d2b17dd558280732
 
 name:           graphql
-version:        0.8.0.0
+version:        0.9.0.0
 synopsis:       Haskell GraphQL implementation
 description:    This package provides a rudimentary parser for the <https://graphql.github.io/graphql-spec/June2018/ GraphQL> language.
 category:       Language
diff --git a/package.yaml b/package.yaml
index d24e4ba..dcdc6c8 100644
--- a/package.yaml
+++ b/package.yaml
@@ -1,5 +1,5 @@
 name:                graphql
-version:             0.8.0.0
+version:             0.9.0.0
 synopsis:            Haskell GraphQL implementation
 description:
   This package provides a rudimentary parser for the
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
diff --git a/stack.yaml b/stack.yaml
index 513705a..a494b98 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -1,4 +1,4 @@
-resolver: lts-16.5
+resolver: lts-16.6
 
 packages:
 - .