Use Seq as base monad in the validator

It is more natural to implement the logic: try to apply each rule to
each node.

src/Language/GraphQL/AST/Document.hs

@@ -49,7 +49,7 @@ import Data.Int (Int32)
 import Data.List.NonEmpty (NonEmpty)
 import Data.Text (Text)
 import qualified Data.Text as Text
-import Language.GraphQL.AST.DirectiveLocation
+import Language.GraphQL.AST.DirectiveLocation (DirectiveLocation)
 
 -- * Language
 
@@ -126,7 +126,7 @@ data Selection
     | InlineFragmentSelection InlineFragment
     deriving (Eq, Show)
 
--- The only required property of a field is its name. Optionally it can also
+-- | The only required property of a field is its name. Optionally it can also
 -- have an alias, arguments, directives and a list of subfields.
 --
 -- In the following query "user" is a field with two subfields, "id" and "name":
@@ -143,7 +143,7 @@ data Field =
     Field (Maybe Name) Name [Argument] [Directive] SelectionSetOpt Location
     deriving (Eq, Show)
 
--- Inline fragments don't have any name and the type condition ("on UserType")
+-- | Inline fragments don't have any name and the type condition ("on UserType")
 -- is optional.
 --
 -- @
@@ -159,7 +159,7 @@ data InlineFragment = InlineFragment
     (Maybe TypeCondition) [Directive] SelectionSet Location
     deriving (Eq, Show)
 
--- A fragment spread refers to a fragment defined outside the operation and is
+-- | A fragment spread refers to a fragment defined outside the operation and is
 -- expanded at the execution time.
 --
 -- @
@@ -190,7 +190,7 @@ data FragmentSpread = FragmentSpread Name [Directive] Location
 -- @
 --
 --  Here "id" is an argument for the field "user" and its value is 4.
-data Argument = Argument Name Value deriving (Eq,Show)
+data Argument = Argument Name Value Location deriving (Eq,Show)
 
 -- ** Fragments
 

src/Language/GraphQL/AST/Encoder.hs

@@ -159,7 +159,7 @@ arguments :: Formatter -> [Argument] -> Lazy.Text
 arguments formatter = parensCommas formatter $ argument formatter
 
 argument :: Formatter -> Argument -> Lazy.Text
-argument formatter (Argument name value')
+argument formatter (Argument name value' _)
     = Lazy.Text.fromStrict name
     <> colon formatter
     <> value formatter value'

src/Language/GraphQL/AST/Lexer.hs

@@ -100,8 +100,8 @@ amp :: Parser T.Text
 amp = symbol "&"
 
 -- | Parser for ":".
-colon :: Parser T.Text
-colon = symbol ":"
+colon :: Parser ()
+colon = symbol ":" >> pure ()
 
 -- | Parser for "=".
 equals :: Parser T.Text

src/Language/GraphQL/AST/Parser.hs

@@ -398,7 +398,12 @@ arguments :: Parser [Argument]
 arguments = listOptIn parens argument <?> "Arguments"
 
 argument :: Parser Argument
-argument = Argument <$> name <* colon <*> value <?> "Argument"
+argument = label "Argument" $ do
+    location <- getLocation
+    name' <- name
+    colon
+    value' <- value
+    pure $ Argument name' value' location
 
 fragmentSpread :: Parser FragmentSpread
 fragmentSpread = label "FragmentSpread" $ do