diff --git a/src/Language/GraphQL/AST/Encoder.hs b/src/Language/GraphQL/AST/Encoder.hs
index eb865c9..9b293bf 100644
--- a/src/Language/GraphQL/AST/Encoder.hs
+++ b/src/Language/GraphQL/AST/Encoder.hs
@@ -15,7 +15,6 @@ module Language.GraphQL.AST.Encoder
 
 import Data.Char (ord)
 import Data.Foldable (fold)
-import Data.Monoid ((<>))
 import qualified Data.List.NonEmpty as NonEmpty
 import Data.Text (Text)
 import qualified Data.Text as Text
diff --git a/src/Language/GraphQL/Error.hs b/src/Language/GraphQL/Error.hs
index 41a2a9c..91911b7 100644
--- a/src/Language/GraphQL/Error.hs
+++ b/src/Language/GraphQL/Error.hs
@@ -20,18 +20,20 @@ import Control.Monad.Trans.State ( StateT
                                  , modify
                                  , runStateT
                                  )
-import Text.Megaparsec ( ParseErrorBundle(..)
-                       , SourcePos(..)
-                       , errorOffset
-                       , parseErrorTextPretty
-                       , reachOffset
-                       , unPos
-                       )
+import Text.Megaparsec
+    ( ParseErrorBundle(..)
+    , PosState(..)
+    , SourcePos(..)
+    , errorOffset
+    , parseErrorTextPretty
+    , reachOffset
+    , unPos
+    )
 
 -- | Wraps a parse error into a list of errors.
 parseError :: Applicative f => ParseErrorBundle Text Void -> f Aeson.Value
 parseError ParseErrorBundle{..}  =
-  pure $ Aeson.object [("errors", Aeson.toJSON $ fst $ foldl go ([], bundlePosState) bundleErrors)]
+    pure $ Aeson.object [("errors", Aeson.toJSON $ fst $ foldl go ([], bundlePosState) bundleErrors)]
   where
     errorObject s SourcePos{..} = Aeson.object
         [ ("message", Aeson.toJSON $ init $ parseErrorTextPretty s)
@@ -39,7 +41,8 @@ parseError ParseErrorBundle{..}  =
         , ("column", Aeson.toJSON $ unPos sourceColumn)
         ]
     go (result, state) x =
-        let (sourcePosition, _, newState) = reachOffset (errorOffset x) state
+        let (_, newState) = reachOffset (errorOffset x) state
+            sourcePosition = pstateSourcePos newState
          in (errorObject x sourcePosition : result, newState)
 
 -- | A wrapper to pass error messages around.
diff --git a/stack.yaml b/stack.yaml
index 18f2dd6..b9bd0ab 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -1,4 +1,4 @@
-resolver: lts-14.25
+resolver: lts-15.0
 
 packages:
 - .
diff --git a/tests/Test/StarWars/Data.hs b/tests/Test/StarWars/Data.hs
index 0318d78..9466991 100644
--- a/tests/Test/StarWars/Data.hs
+++ b/tests/Test/StarWars/Data.hs
@@ -16,7 +16,6 @@ module Test.StarWars.Data
     , typeName
     ) where
 
-import Data.Monoid (mempty)
 import Data.Functor.Identity (Identity)
 import Control.Applicative (Alternative(..), liftA2)
 import Control.Monad.Trans.Except (throwE)