1 files changed, 2 insertions, 0 deletions
diff --git a/lib/LintWriter.hs b/lib/LintWriter.hs
index fa8207b..5a6fc7d 100644
--- a/lib/LintWriter.hs
+++ b/lib/LintWriter.hs
@@ -7,6 +7,7 @@
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE RankNTypes        #-}
 {-# LANGUAGE TupleSections     #-}
+{-# LANGUAGE DeriveFunctor     #-}
 {-# OPTIONS_GHC -Wno-missing-signatures #-}
 
 -- | a monad that collects warnings, outputs, etc,
@@ -67,6 +68,7 @@ type LintWriter' ctxt res =
 -- | it already collected.
 newtype LinterState ctxt = LinterState
   { fromLinterState :: ([Lint], ctxt)}
+  deriving Functor
 
 -- | The result of running a linter: an adjusted context, and a list of lints.
 -- | This is actually just a type synonym of LinterState, but kept seperately