Draft the Validation API

src/Language/GraphQL/Validate/Rules.hs

@@ -0,0 +1,25 @@
+{- 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/. -}
+
+module Language.GraphQL.Validate.Rules
+    ( Rule(..)
+    , executableDefinitionsRule
+    , specifiedRules
+    ) where
+
+import Language.GraphQL.AST.Document
+
+newtype Rule
+    = DefinitionRule (Definition -> Maybe String)
+
+specifiedRules :: [Rule]
+specifiedRules =
+    [ executableDefinitionsRule
+    ]
+
+executableDefinitionsRule :: Rule
+executableDefinitionsRule = DefinitionRule go
+  where
+    go (ExecutableDefinition _definition _) = Nothing
+    go _ = Just "Definition must be OperationDefinition or FragmentDefinition."