diff --git a/creatur.cabal b/creatur.cabal
--- a/creatur.cabal
+++ b/creatur.cabal
@@ -1,5 +1,5 @@
 Name:              creatur
-Version:           5.7.2
+Version:           5.7.3
 Stability:         experimental
 Synopsis:          Framework for artificial life experiments.
 Description:       A software framework for automating experiments
@@ -36,7 +36,7 @@
 source-repository this
   type:     git
   location: https://github.com/mhwombat/creatur.git
-  tag:      5.7.2
+  tag:      5.7.3
 
 library
   GHC-Options:      -Wall -fno-warn-orphans
diff --git a/src/ALife/Creatur/Persistent.hs b/src/ALife/Creatur/Persistent.hs
--- a/src/ALife/Creatur/Persistent.hs
+++ b/src/ALife/Creatur/Persistent.hs
@@ -15,7 +15,9 @@
     Persistent,
     mkPersistent,
     getPS,
-    putPS
+    putPS,
+    modifyPS,
+    runPS
   ) where
 
 import ALife.Creatur.Util (modifyLift)
@@ -47,6 +49,16 @@
   modify (\p -> p { psValue=s })
   p' <- get
   liftIO $ store p'
+
+modifyPS :: (Show a, Read a) => (a -> a) -> StateT (Persistent a) IO ()
+modifyPS f = do
+  p <- getPS
+  putPS $ f p
+
+runPS :: Read a => (a -> b) -> StateT (Persistent a) IO b
+runPS f = do
+  p <- getPS
+  return $ f p
 
 store :: Show a => Persistent a -> IO ()
 store p = do