small example for space-efficiency via lazy evaluation?
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));
On 2018-09-04 12:36 PM, Johannes Waldmann wrote:
main = print $ sum $ map (^ 2) $ [ 1 :: Int .. 10^8 ] [...] So, my question is, what do you use as a (teaching) example for space-efficiency via lazy evaluation?
I would skip the summing. Print the whole list. Sure it would take forever to finish, but during that time I would also fire up htop or something to show how much memory the process doesn't use as it progresses. And change Int to Integer and bump up the upper bound to 10^12 or something --- or even have no upper bound at all. And point out how the printing starts right away as opposed to "waiting for the whole list to be completely built before printing begins". And for the sake of engagement, before running the experiment, invite the students to make predictions about how much memory, how it grows, how much time before the printing begins, etc. Learning does not happen by nodding. Learn happens by dropping your jaw all the time. In my class I used these two other examples (because I didn't want to do I/O yet): doITerminate = take 2 (foo 0) where foo n = n : foo (n + 1) doIEvenMakeSense = take 2 foo where foo = 0 : foo They're merely "take 2" because next I also had to showed the detailed steps of lazy evaluation. It would be boring to go "take 10".
participants (2)
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Albert Y. C. Lai -
Johannes Waldmann