diff --git a/src/GEval/Core.hs b/src/GEval/Core.hs
index e13ac23..10153ac 100644
--- a/src/GEval/Core.hs
+++ b/src/GEval/Core.hs
@@ -266,7 +266,7 @@ extensionsHandled = ["tsv", "jsonl"]
 data LineSource m = LineSource (ConduitT () Text m ()) (Text -> ItemTarget) (Text -> Text) SourceSpec Word32
 
 data LineSourcesSpecification m = LineSourcesSpecification {
-  lineSourcesFilter :: Maybe (Text -> Bool),
+  lineSourcesFilter :: Filter,
   lineSourcesInputSource :: LineSource m,
   lineSourcesExpectedSource :: LineSource m,
   lineSourcesOutputSource :: LineSource m }
@@ -377,7 +377,7 @@ checkAndGetDataSources forceInput gevalSpec = do
         challengeDataSourceExpected = expectedSource,
         challengeDataSourceSelector = mSelector,
         challengeDataSourcePreprocess = preprocess,
-        challengeDataSourceFilter = Nothing,
+        challengeDataSourceFilter = Filter Nothing,
         challengeDataSourceInHeader = mInHeader,
         challengeDataSourceOutHeader = mOutHeader }
 
@@ -502,7 +502,7 @@ gevalCoreOnSingleLines metric preprocess inpDecoder inpLine expDecoder expLine o
                            then preprocess
                            else id
         lsSpec = LineSourcesSpecification {
-          lineSourcesFilter = Nothing,
+          lineSourcesFilter = Filter Nothing,
           lineSourcesInputSource = singleLineAsLineSource inpLine inpDecoder preprocess,
           lineSourcesExpectedSource = singleLineAsLineSource expLine expDecoder outputPreprocess,
           lineSourcesOutputSource = singleLineAsLineSource outLine outDecoder outputPreprocess }
diff --git a/src/GEval/DataSource.hs b/src/GEval/DataSource.hs
index 9df6912..ee12555 100644
--- a/src/GEval/DataSource.hs
+++ b/src/GEval/DataSource.hs
@@ -1,6 +1,7 @@
 module GEval.DataSource
   (ChallengeDataSource(..),
-   DataSource(..))
+   DataSource(..),
+   Filter(..))
   where
 
 import Data.Text
@@ -9,6 +10,8 @@ import Data.Conduit.SmartSource
 import Data.Conduit.Header
 import GEval.Selector
 
+newtype Filter = Filter (Maybe (Text -> Bool))
+
 -- | This type specifies the way the challenge data (input and
 -- expected data, but not outputs) flow into evaluation.
 --
@@ -18,7 +21,7 @@ data ChallengeDataSource = ChallengeDataSource {
   challengeDataSourceExpected :: SourceSpec,
   challengeDataSourceSelector :: Maybe Selector,
   challengeDataSourcePreprocess :: Text -> Text,
-  challengeDataSourceFilter :: Maybe (Text -> Bool),
+  challengeDataSourceFilter :: Filter,
   challengeDataSourceInHeader :: Maybe TabularHeader,
   challengeDataSourceOutHeader :: Maybe TabularHeader }