diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+1.5.18
+
+* Fix space leak
+
 1.5.17
 
 * Add `optionsExt`: Extended version of `options` with header, footer,
diff --git a/src/Turtle/Shell.hs b/src/Turtle/Shell.hs
--- a/src/Turtle/Shell.hs
+++ b/src/Turtle/Shell.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE BangPatterns              #-}
 {-# LANGUAGE CPP                       #-}
 {-# LANGUAGE ExistentialQuantification #-}
 {-# LANGUAGE RankNTypes                #-}
@@ -101,21 +102,23 @@
 -- | A @(Shell a)@ is a protected stream of @a@'s with side effects
 newtype Shell a = Shell { _foldShell:: forall r . FoldShell a r -> IO r }
 
+data Maybe' a = Just' !a | Nothing'
+
 translate :: FoldM IO a b -> FoldShell a b
-translate (FoldM step begin done) = FoldShell step' Nothing done'
+translate (FoldM step begin done) = FoldShell step' Nothing' done'
   where
-    step' Nothing a = do
+    step' Nothing' a = do
         x  <- begin
         x' <- step x a
-        return (Just x')
-    step' (Just x) a = do
+        return $! Just' x'
+    step' (Just' x) a = do
         x' <- step x a
-        return (Just x')
+        return $! Just' x'
 
-    done' Nothing = do
+    done' Nothing' = do
         x <- begin
         done x
-    done' (Just x) = do
+    done' (Just' x) = do
         done x
 
 -- | Use a @`FoldM` `IO`@ to reduce the stream of @a@'s produced by a `Shell`
diff --git a/turtle.cabal b/turtle.cabal
--- a/turtle.cabal
+++ b/turtle.cabal
@@ -1,5 +1,5 @@
 Name: turtle
-Version: 1.5.17
+Version: 1.5.18
 Cabal-Version: >=1.10
 Build-Type: Simple
 License: BSD3