diff --git a/cbits/store.c b/cbits/store.c
--- a/cbits/store.c
+++ b/cbits/store.c
@@ -1,4 +1,5 @@
 #include <stdlib.h>
+#include <memory.h>
 #include <stdint.h>
 
 void **values = NULL;
@@ -24,15 +25,29 @@
   return index;
 }
 
+void x_set(uint32_t index,void*value) {
+  if (index + 1 > size) {
+    size_t new_size = index + 1;
+    values = realloc(values,new_size * sizeof(void*));
+    memset(values + size,0,new_size - size);
+    size = new_size;
+  }
+  values[index] = value;
+}
+
 void *x_get(uint32_t index) {
-  return values[index];
+  if (index >= 0 && index < size) {
+    return values[index];
+  } else {
+    return NULL;
+  }
 }
 
 uint32_t x_lookup(uint32_t index) {
   if (values == NULL)
     return 0;
   else {
-    if (index < size) {
+    if (index >= 0 && index < size) {
       return values[index] == NULL? 0 : 1;
     } else {
       return 0;
diff --git a/foreign-store.cabal b/foreign-store.cabal
--- a/foreign-store.cabal
+++ b/foreign-store.cabal
@@ -1,5 +1,5 @@
 name:                foreign-store
-version:             0.0
+version:             0.1
 synopsis:            Store a stable pointer in a foreign context to be retrieved later. Persists through GHCi reloads.
 description:         Store a stable pointer in a foreign context to be retrieved later. Persists through GHCi reloads.
 license:             BSD3
@@ -10,6 +10,7 @@
 category:            Development
 build-type:          Simple
 cabal-version:       >=1.8
+Homepage:            https://github.com/chrisdone/foreign-store
 
 library
   hs-source-dirs:    src/
@@ -18,3 +19,7 @@
   build-depends:     base >= 4 && <5
   c-sources:         cbits/store.c
   include-dirs:      cbits
+
+source-repository head
+  type:     git
+  location: git://github.com/chrisdone/foreign-store.git
diff --git a/src/Foreign/Store.hs b/src/Foreign/Store.hs
--- a/src/Foreign/Store.hs
+++ b/src/Foreign/Store.hs
@@ -2,13 +2,17 @@
 {-# LANGUAGE ForeignFunctionInterface #-}
 
 -- | Store a stable pointer in a foreign context to be retrieved
--- later. Persists through GHCi reloads.
+-- later. Persists through GHCi reloads. Not thread-safe.
 
 module Foreign.Store
-  (newStore
+  (-- * Foreign stores
+   writeStore
+  ,newStore
   ,lookupStore
   ,readStore
   ,deleteStore
+  ,storeAction
+  ,withStore
   ,Store(..))
   where
 
@@ -18,6 +22,7 @@
 import Foreign.Ptr
 import Foreign.StablePtr
 
+-- | An exception when working with stores.
 data StoreException
   = StoreNotFound
   deriving (Show,Eq,Typeable)
@@ -29,7 +34,7 @@
   Store Word32
   deriving (Show,Eq)
 
--- | Lookup from the store.
+-- | Lookup from the store if an index is allocated.
 lookupStore :: Word32 -> IO (Maybe (Store a))
 lookupStore i =
   do r <- x_lookup i
@@ -37,17 +42,32 @@
         then return Nothing
         else return (Just (Store i))
 
--- | Make a new store. The internal vector of stores grows in
--- side. When stores are deleted the vector does not shrink, but old
--- slots are re-used.
+-- | Allocates or finds an unallocated store. The index is random. The
+-- internal vector of stores grows in size. When stores are deleted
+-- the vector does not shrink, but old slots are re-used.
+-- Not thread-safe.
 newStore :: a -> IO (Store a)
 newStore a =
   do sptr <- newStablePtr a
      i <- x_store sptr
      return (Store i)
 
--- | Read from the store. If the store has been deleted, this will
--- throw an exception.
+-- | Write to the store at the given index. If a store doesn't exist,
+-- creates one and resizes the store vector to fit. If there is
+-- already a store at the given index, deletes that store with
+-- 'deleteStore' before replacing it.
+-- Not thread-safe.
+writeStore :: Store a -> a -> IO ()
+writeStore (Store i) a =
+  do existing <- lookupStore i
+     maybe (return ()) deleteStore existing
+     sptr <- newStablePtr a
+     x_set i sptr
+     return ()
+
+-- | Read from the store. If the store has been deleted or is
+-- unallocated, this will throw an exception.
+-- Not thread-safe.
 readStore :: Store a -> IO a
 readStore (Store i) =
   do sptr <- x_get i
@@ -57,6 +77,7 @@
 
 -- | Frees the stable pointer for GC and frees up the slot in the
 -- store. Deleting an already deleted store is a no-op.
+-- Not thread-safe.
 deleteStore :: Store a -> IO ()
 deleteStore (Store i) = do
   sptr <- x_get i
@@ -65,9 +86,26 @@
      else do freeStablePtr sptr
              x_delete i
 
+-- | Run the action and store the result.
+storeAction :: Store a -> IO a -> IO a
+storeAction s m =
+  do v <- m
+     writeStore s v
+     return v
+
+-- | Run the action and store the result.
+withStore :: Store a -> (a -> IO b) -> IO b
+withStore s f =
+  do v <- readStore s
+     f v
+
 foreign import ccall
   "x-helpers.h x_store"
   x_store :: StablePtr a -> IO Word32
+
+foreign import ccall
+  "x-helpers.h x_set"
+  x_set :: Word32 -> StablePtr a -> IO ()
 
 foreign import ccall
   "x-helpers.h x_get"