Define a Command type.

src/Command.hs

@@ -1,6 +1,15 @@
+{-# LANGUAGE GADTs #-}
 module Command
 ( module C
 ) where
 
 import Command.Diff as C
 import Command.Parse as C
+import Control.Monad.Free.Freer
+import Prologue
+import Source
+
+data CommandF f where
+  ReadFile :: FilePath -> CommandF SourceBlob
+
+type Command = Freer CommandF