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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/. -}
+
+-- | Definitions used by the validation rules and the validator itself.
+module Language.GraphQL.Validate.Validation
+    ( Error(..)
+    , Path(..)
+    , Rule(..)
+    , RuleT
+    , Validation(..)
+    ) where
+
+import Control.Monad.Trans.Reader (ReaderT(..))
+import Data.HashMap.Strict (HashMap)
+import Data.Text (Text)
+import Language.GraphQL.AST.Document
+import Language.GraphQL.Type.Schema (Schema)
+import qualified Language.GraphQL.Type.Schema as Schema
+
+-- | 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 -- ^ 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)
+
+-- | Validation rule context.
+data Validation m = Validation
+    { ast :: Document
+    , schema :: Schema m
+    , types :: HashMap Name (Schema.Type m)
+    , rules :: [Rule m]
+    }
+
+-- | '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.
+data Rule m
+    = DefinitionRule (Definition -> RuleT m)
+    | OperationDefinitionRule (OperationDefinition -> RuleT m)
+    | FragmentDefinitionRule (FragmentDefinition -> RuleT m)
+
+-- | Monad transformer used by the rules.
+type RuleT m = ReaderT (Validation m) Maybe Error