diff --git a/ref-extras.cabal b/ref-extras.cabal
--- a/ref-extras.cabal
+++ b/ref-extras.cabal
@@ -4,10 +4,10 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 9e81744739214454e5314bea860635661caf7b4f4296bb4c6f873e105c986c45
+-- hash: 0c53a1b53092126beb62eea99bfb47ad42c02b42aef27681ee3434592b29b682
 
 name:           ref-extras
-version:        0.1.0
+version:        0.1.1
 synopsis:       Extra stuff for mutable references
 description:    Please see the README on GitHub at <https://github.com/ejconlon/ref-extras#readme>
 category:       Data
@@ -32,6 +32,7 @@
       RefExtras.Classes
       RefExtras.EVar
       RefExtras.LensRef
+      RefExtras.Memo
       RefExtras.SomeRef
       RefExtras.XVar
   other-modules:
@@ -42,9 +43,11 @@
   ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wpartial-fields -Wredundant-constraints -fno-warn-unused-top-binds
   build-depends:
       base >=4.12 && <5
+    , hashable >=1.3
     , little-rio >=0.1.1
     , microlens >=0.4
     , stm >=2.5
     , unliftio >=0.2
     , unliftio-core >=0.1
+    , unordered-containers >=0.2
   default-language: Haskell2010
diff --git a/src/RefExtras/Memo.hs b/src/RefExtras/Memo.hs
new file mode 100644
--- /dev/null
+++ b/src/RefExtras/Memo.hs
@@ -0,0 +1,68 @@
+module RefExtras.Memo
+  ( Memo
+  , accessMemo
+  , clearMemo
+  , freezeMemo
+  , newMemo
+  , thawMemo
+  , tryReadMemo
+  , writeMemo
+  ) where
+
+import Control.Monad (join)
+import Control.Monad.IO.Class (MonadIO)
+import Control.Monad.IO.Unlift (MonadUnliftIO)
+import Data.Hashable (Hashable)
+import Data.HashMap.Strict (HashMap)
+import qualified Data.HashMap.Strict as HashMap
+import Prelude
+import RefExtras.EVar (EVar, accessEVar, newEventualEVar, newReadyEVar, tryReadEVar, writeEVar)
+import RefExtras.XVar (XVar, newXVar, readXVar, splitXVar, writeXVar)
+
+newtype Memo k a = Memo { unMemo :: XVar (HashMap k (EVar a)) }
+
+newMemo :: MonadIO m => m (Memo k a)
+newMemo = fmap Memo (newXVar HashMap.empty)
+
+clearMemo :: MonadIO m => Memo k a -> m ()
+clearMemo (Memo v) = writeXVar v HashMap.empty
+
+accessMemo :: (MonadUnliftIO m, Eq k, Hashable k) => Memo k a -> k -> m a -> m a
+accessMemo (Memo v) k act = splitXVar v $ \m write ->
+  case HashMap.lookup k m of
+    Just w -> write m >> accessEVar w act
+    Nothing -> newEventualEVar (\w -> write (HashMap.insert k w m)) act
+
+tryReadMemo :: (MonadIO m, Eq k, Hashable k) => Memo k a -> k -> m (Maybe a)
+tryReadMemo (Memo v) k = do
+  m <- readXVar v
+  case HashMap.lookup k m of
+    Nothing -> pure Nothing
+    Just w -> tryReadEVar w
+
+-- Blocks if k is being populated
+writeMemo :: (MonadUnliftIO m, Eq k, Hashable k) => Memo k a -> k -> a -> m ()
+writeMemo (Memo v) k a = splitXVar v $ \m write ->
+  case HashMap.lookup k m of
+    Just w -> write m >> writeEVar w a
+    Nothing -> do
+      w <- newReadyEVar a
+      write (HashMap.insert k w m)
+
+bindFor :: (Monad t, Traversable t, Applicative f) => t a -> (a -> f (t b)) -> f (t b)
+bindFor t f = fmap join (traverse f t)
+
+freezeMemo :: (MonadIO m, Eq k, Hashable k) => Memo k a -> m (HashMap k a)
+freezeMemo (Memo v) = do
+  m <- readXVar v
+  fmap HashMap.fromList $ bindFor (HashMap.toList m) $ \(k, w) -> do
+    ma <- tryReadEVar w
+    case ma of
+      Just a -> pure [(k, a)]
+      _ -> pure []
+
+thawMemo :: MonadIO m => HashMap k a -> m (Memo k a)
+thawMemo m = do
+  n <- traverse newReadyEVar m
+  v <- newXVar n
+  pure (Memo v)