Validate variable names are unique

src/Language/GraphQL/AST/Document.hs

@@ -253,7 +253,8 @@ data ObjectField a = ObjectField Name a
 --
 -- Variables are usually passed along with the query, but not in the query
 -- itself. They make queries reusable.
-data VariableDefinition = VariableDefinition Name Type (Maybe ConstValue)
+data VariableDefinition =
+    VariableDefinition Name Type (Maybe ConstValue) Location
     deriving (Eq, Show)
 
 -- ** Type References

src/Language/GraphQL/AST/Encoder.hs

@@ -95,7 +95,7 @@ variableDefinitions formatter
     = parensCommas formatter $ variableDefinition formatter
 
 variableDefinition :: Formatter -> VariableDefinition -> Lazy.Text
-variableDefinition formatter (VariableDefinition var ty defaultValue')
+variableDefinition formatter (VariableDefinition var ty defaultValue' _)
     = variable var
     <> eitherFormat formatter ": " ":"
     <> type' ty

src/Language/GraphQL/AST/Parser.hs

@@ -492,12 +492,13 @@ variableDefinitions = listOptIn parens variableDefinition
     <?> "VariableDefinitions"
 
 variableDefinition :: Parser VariableDefinition
-variableDefinition = VariableDefinition
-    <$> variable
-    <*  colon
-    <*> type'
-    <*> defaultValue
-    <?> "VariableDefinition"
+variableDefinition = label "VariableDefinition" $ do
+    location <- getLocation
+    variableName <- variable
+    colon
+    variableType <- type'
+    variableValue <- defaultValue
+    pure $ VariableDefinition variableName variableType variableValue location
 
 variable :: Parser Name
 variable = dollar *> name <?> "Variable"