diff --git a/byg/src/Evaluation/FunctionIO.hs b/byg/src/Evaluation/FunctionIO.hs
index a9072df..4104aa7 100644
--- a/byg/src/Evaluation/FunctionIO.hs
+++ b/byg/src/Evaluation/FunctionIO.hs
@@ -1,11 +1,13 @@
 module Evaluation.FunctionIO
   ( evalFunctionIO
+  , functionIOTouchesFilesystem
   ) where
 
 import Prelude hiding (String, FilePath)
 
 import Types.Values
-import Types (FunctionIO(..), Value(..), toValue, makeImage)
+import Types.FunctionIO
+import Types.Value (Value(..), toValue, makeImage)
 
 import qualified Data.Text.IO as T
 import qualified Codec.Picture as CP
@@ -46,3 +48,14 @@ evalFunctionIO f x = case (f, x) of
 
   _ ->
     error ("unexpected combination of function and argument type; got function " ++ show f ++ " with argument " ++ show x)
+
+functionIOTouchesFilesystem :: FunctionIO -> Bool
+functionIOTouchesFilesystem = \case
+  ListDirectory -> False
+  IsDirectory -> False
+  ReadTextFile -> False
+  OpenImage -> False
+  SaveImage -> True
+  SaveTextFile -> True
+  CopyFile -> True
+  MakeDir -> True
diff --git a/byg/src/Types/Dependency.hs b/byg/src/Types/Dependency.hs
index da4fbc3..9dee251 100644
--- a/byg/src/Types/Dependency.hs
+++ b/byg/src/Types/Dependency.hs
@@ -5,6 +5,7 @@ module Types.Dependency
   , Dependency(..)
   , makeDependency
   , makeUToken
+  , actionTouchesFilesystem
   , formatDependencyTrees
   ) where
 
@@ -12,6 +13,7 @@ import Types.Token (Token(..))
 import Types.Value (Value)
 import Types.Function (Function)
 import Types.FunctionIO (FunctionIO)
+import Evaluation.FunctionIO (functionIOTouchesFilesystem)
 
 import Text.Printf (printf)
 import Data.Text (Text)
@@ -50,6 +52,20 @@ makeUToken = \case
   ListToken ts -> UListToken (map makeUToken ts)
   NoToken -> UNoToken
 
+actionTouchesFilesystem :: Action -> Bool
+actionTouchesFilesystem = \case
+  Function _ -> False
+  FunctionIO f -> functionIOTouchesFilesystem f
+  Inject _ -> False
+  FilterComp -> False
+  UntupleFst -> False
+  UntupleSnd -> False
+  UnzipFst -> False
+  UnzipSnd -> False
+  MapComp subDeps _ _ -> any dependencyTouchesFilesystem subDeps
+  where dependencyTouchesFilesystem (Dependency _ action _) =
+          actionTouchesFilesystem action
+
 formatDependencyTrees :: [Dependency] -> Text
 formatDependencyTrees = T.concat . (formatDependencyTrees' "")
   where formatDependencyTrees' indentation = concatMap (formatDependencyTree indentation)