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diff --git a/src/Language/GraphQL/Validate/Rules.hs b/src/Language/GraphQL/Validate/Rules.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/. -}
+
+-- | This module contains default rules defined in the GraphQL specification.
+module Language.GraphQL.Validate.Rules
+    ( Rule(..)
+    , executableDefinitionsRule
+    , specifiedRules
+    ) where
+
+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
+    go (ExecutableDefinition _definition _) = Nothing
+    go _ = Just "Definition must be OperationDefinition or FragmentDefinition."