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diff --git a/src/Language/GraphQL/Validate.hs b/src/Language/GraphQL/Validate.hs
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+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   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/. -}
+
+{-# LANGUAGE ExplicitForAll #-}
+{-# LANGUAGE LambdaCase #-}
+
+module Language.GraphQL.Validate
+    ( Error(..)
+    , Path(..)
+    , document
+    , module Language.GraphQL.Validate.Rules
+    ) where
+
+import Control.Monad.Trans.Reader (Reader, asks, runReader)
+import Data.Foldable (foldrM)
+import Data.Sequence (Seq(..), (><), (|>))
+import qualified Data.Sequence as Seq
+import Data.Text (Text)
+import Language.GraphQL.AST.Document
+import Language.GraphQL.Type.Schema
+import Language.GraphQL.Validate.Rules
+
+data Context m = Context
+    { ast :: Document
+    , schema :: Schema m
+    , rules :: [Rule]
+    }
+
+type ValidateT m = Reader (Context m) (Seq Error)
+
+data Path
+    = Segment Text
+    | Index Int
+    deriving (Eq, Show)
+
+data Error = Error
+    { message :: String
+    , locations :: [Location]
+    , path :: [Path]
+    } deriving (Eq, Show)
+
+document :: forall m. Schema m -> [Rule] -> Document -> Seq Error
+document schema' rules' document' =
+    runReader (foldrM go Seq.empty document') context
+  where
+    context = Context
+        { ast = document'
+        , schema = schema'
+        , rules = rules'
+        }
+    go definition' accumulator = (accumulator ><) <$> definition definition'
+
+definition :: forall m. Definition -> ValidateT m
+definition = \case
+    definition'@(ExecutableDefinition executableDefinition' _) -> do
+        applied <- applyRules definition'
+        children <- executableDefinition executableDefinition'
+        pure $ children >< applied
+    definition' -> applyRules definition'
+  where
+    applyRules definition' = foldr (ruleFilter definition') Seq.empty
+        <$> asks rules
+    ruleFilter definition' (DefinitionRule rule) accumulator
+        | Just message' <- rule definition' =
+            accumulator |> Error
+                { message = message'
+                , locations = [definitionLocation definition']
+                , path = []
+                }
+        | otherwise = accumulator
+    definitionLocation (ExecutableDefinition _ location) = location
+    definitionLocation (TypeSystemDefinition _ location) = location
+    definitionLocation (TypeSystemExtension _ location) = location
+
+executableDefinition :: forall m. ExecutableDefinition -> ValidateT m
+executableDefinition (DefinitionOperation definition') =
+    operationDefinition definition'
+executableDefinition (DefinitionFragment definition') =
+    fragmentDefinition definition'
+
+operationDefinition :: forall m. OperationDefinition -> ValidateT m
+operationDefinition (SelectionSet _operation) =
+    pure Seq.empty
+operationDefinition (OperationDefinition _type _name _variables _directives _selection) =
+    pure Seq.empty
+
+fragmentDefinition :: forall m. FragmentDefinition -> ValidateT m
+fragmentDefinition _fragment = pure Seq.empty