Dear Cafe, I wanted to demonstrate that main = print $ sum $ map (^ 2) $ [ 1 :: Int .. 10^8 ] without any optimisations, still runs in constant space because garbage is collected immediately. Except that it does not: ghc -O0 space.hs -rtsopts -ddump-simpl ./space +RTS -M80k -A10k gives me unoptimized Core as expected, but exhausts the heap. After some experimentation, I found that replacing Prelude.sum with Data.Foldable.foldl' (+) 0 achieves what I want. I think the reason is that instance Foldable [] implements sum by foldl (non-strict). Both versions will run without any allocation as soon as we compile with -O1 . So, my question is, what do you use as a (teaching) example for space-efficiency via lazy evaluation? Preferrably a one-liner, using only standard libraries, and such that the effect is not rendered moot by -O2. - J PS: It is magic that foldl and foldl' produce identical core here? $wgo_s5we (w_s5w8 :: GHC.Prim.Int#) (ww1_s5wc :: GHC.Prim.Int#) = case GHC.Prim.==# w_s5w8 ww_s5w5 of { __DEFAULT -> jump $wgo_s5we (GHC.Prim.+# w_s5w8 1#) (GHC.Prim.+# ww1_s5wc (GHC.Prim.*# w_s5w8 w_s5w8));