Put sources functionality in its own file

byg/src/Sources.hs

@@ -0,0 +1,16 @@
+{-# LANGUAGE FunctionalDependencies #-}
+module Sources where
+
+import Data.ByteString (ByteString)
+
+data Source a where
+  Data :: a -> Source a
+
+class SourceState a where
+  stateOfSource :: a -> IO ByteString
+
+instance SourceState FilePath where
+  stateOfSource = undefined
+
+class SourceState a => IsFIO f a b | f -> a b where
+  runFIO :: f -> a -> IO b