diff --git a/src/Data/Array/ST/Iterator.purs b/src/Data/Array/ST/Iterator.purs
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+++ b/src/Data/Array/ST/Iterator.purs
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+module Data.Array.ST.Iterator
+  ( Iterator
+  , iterator
+  , next
+  , peek
+  , exhausted
+  , pushWhile
+  , pushAll
+  ) where
+
+import Prelude
+import Control.Monad.Eff (Eff, whileE)
+import Control.Monad.ST (ST, STRef, newSTRef, readSTRef, writeSTRef, modifySTRef)
+import Data.Array.ST (STArray, pushSTArray)
+
+import Data.Maybe (Maybe(..), isNothing)
+
+-- | This type provides a slightly easier way of iterating over an array's
+-- | elements in an STArray computation, without having to keep track of
+-- | indices.
+data Iterator h a = Iterator (Int -> Maybe a) (STRef h Int)
+
+-- | Make an Iterator given an indexing function into an array (or anything
+-- | else).
+iterator :: forall a h r. (Int -> Maybe a) -> Eff (st :: ST h | r) (Iterator h a)
+iterator f =
+  Iterator f <$> newSTRef 0
+
+-- | Perform an action once for each item left in an iterator.
+iterate :: forall a h r. Iterator h a -> (a -> Eff (st :: ST h | r) Unit) -> Eff (st :: ST h | r) Unit
+iterate iter f = do
+  break <- newSTRef false
+  whileE (not <$> readSTRef break) do
+    mx <- next iter
+    case mx of
+      Just x -> f x
+      Nothing -> void $ writeSTRef break true
+
+-- | Get the next item out of an iterator, advancing it. Returns Nothing if the
+-- | Iterator is exhausted.
+next :: forall a h r. Iterator h a -> Eff (st :: ST h | r) (Maybe a)
+next (Iterator f currentIndex) = do
+  i <- readSTRef currentIndex
+  modifySTRef currentIndex (_ + 1)
+  pure (f i)
+
+-- | Get the next item out of an iterator without advancing it.
+peek :: forall a h r. Iterator h a -> Eff (st :: ST h | r) (Maybe a)
+peek (Iterator f currentIndex) = do
+  i <- readSTRef currentIndex
+  pure (f i)
+
+-- | Check whether an iterator has been exhausted.
+exhausted :: forall a h r. Iterator h a -> Eff (st :: ST h | r) Boolean
+exhausted = map isNothing <<< peek
+
+-- | Extract elements from an iterator and push them on to an STArray for as
+-- | long as those elements satisfy a given predicate.
+pushWhile :: forall a h r. (a -> Boolean) -> Iterator h a -> STArray h a -> Eff (st :: ST h | r) Unit
+pushWhile p iter array = do
+  break <- newSTRef false
+  whileE (not <$> readSTRef break) do
+    mx <- peek iter
+    case mx of
+      Just x | p x -> do
+        pushSTArray array x
+        void $ next iter
+      _ ->
+        void $ writeSTRef break true
+
+-- | Push the entire remaining contents of an iterator onto an STArray.
+pushAll :: forall a h r. Iterator h a -> STArray h a -> Eff (st :: ST h | r) Unit
+pushAll = pushWhile (const true)