Validate single root field in subscriptions

src/Language/GraphQL/Validate/Validation.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/. -}
+
+-- | Definitions used by the validation rules and the validator itself.
+module Language.GraphQL.Validate.Validation
+    ( Validation(..)
+    , Rule(..)
+    , RuleT
+    ) where
+
+import Control.Monad.Trans.Reader (ReaderT(..))
+import Data.HashMap.Strict (HashMap)
+import Language.GraphQL.AST.Document
+import Language.GraphQL.Type.Schema (Schema)
+import qualified Language.GraphQL.Type.Schema as Schema
+
+-- | 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)
+
+-- | Monad transformer used by the rules.
+type RuleT m = ReaderT (Validation m) Maybe String