#ocaml on 2018-12-01 — irc logs at freenode.irclog.whitequark.org

2018-10-19 11:39 adrien changed the topic of #ocaml to: Discussions about the OCaml programming language | http://www.ocaml.org | OCaml 4.07.1 release notes: https://caml.inria.fr/pub/distrib/ocaml-4.07/notes/Changes | Try OCaml in your browser: http://try.ocamlpro.com | Public channel logs at http://irclog.whitequark.org/ocaml | Due to ongoing spam, you must register your nickname to talk on the channel

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09:23 <Desetude> Hi all, I have the following function: https://gist.github.com/Desetude/99b9d4846860484c66a967adcaaca3ee (I know it's terrible, it's the first function I've created in Ocaml). But I get "This expression has type 'a list but an expression was expected of type int" in the last argument of the recursive call on the last line

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09:50 <sonologico> Desetude, I just pasted it into utop and it compiles for me. I only get warnings about non exhaustive matches

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09:50 <Desetude> sonologico: As you said that, I found the problem, it's because I had the args in List.mem the wrong way round so the type inferencing was getting confused

09:51 <Desetude> It seems like Base does List.mem the other way round to some other libs? I'm not really sure

09:51 <sonologico> yes, the stdlib one uses the order that in your original code

09:51 <Desetude> ah ok that's a bit annoying

09:52 <sonologico> I just removed the ~equal:(=) to use the stdlib one when I pasted

09:54 <Desetude> ok thanks. Which list is it talking about with the non exhaustive matches?

09:54 <Desetude> or is it both

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10:11 <sonologico> Desetude, both

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10:13 <sonologico> in the function argument list, you're doing something like (h :: t) for both arguments. The warning is saying that you're not considering [] (the empty list)

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10:14 <sonologico> so you'll get an error if your function is called like `find_first_duplicate_sum [] []`

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21:46 <tanelso2> Hello everyone! I am learning OCaml by doing Advent of Code puzzles. I solved the day1 puzzles, but my code takes 30-40 seconds to find the solution and I'm not really sure why.

21:47 <companion_cube> can you show us the code?

21:47 <tanelso2> Yeah https://github.com/tanelso2/advent-of-code-2018/blob/master/day1/day1.ml

21:48 <tanelso2> I am a complete newbie to Ocaml so I'm sure I'm doing something wrong or very inefficiently

21:48 <companion_cube> part1 seems fine

21:49 <tanelso2> Yeah part1 runs quickly

21:49 <tanelso2> It's part2 I had trouble with

21:49 <companion_cube> ah, you shouldn't use Set.exists for that

21:49 <companion_cube> there must be a Set.mem or Set.contains

21:49 <companion_cube> (I don't know Core much, but it's the whole point of Set)

21:50 <companion_cube> that'd turn your O(n) lookup into a O(ln(n))

21:50 <companion_cube> otherwise, the code looks pretty good, except (imho) for the 4 spaces indent

21:50 <companion_cube> you may want to use ocp-indent :)

21:51 <Drup> even if the List version, List.mem is a thing.

21:52 <companion_cube> yeah but it's not very useful since it uses polymorphic equality

21:52 <companion_cube> I'd say it's a code smell in 2018 :)

21:53 <tanelso2> I completely skipped over List.mem while looking through the docs. Alright thanks for the hint

21:53 <tanelso2> Oh there is a Set.mem

21:54 <companion_cube> yes, it's pretty important

21:54 <companion_cube> also, Set.union, etc.

21:54 <tanelso2> Interesting. Why is it called mem btw? I'm used to languages that call it contains or something similar

21:54 <companion_cube> wow ok this does 130,300 iterations for me… didn't realize it was so computational

21:54 <companion_cube> tanelso2: `member`

21:55 <companion_cube> and it's `mem x set` (x member of set) rather than `contains set x`

21:55 <companion_cube> in this particular interface

21:57 <tanelso2> Ah makes sense

21:58 <tanelso2> Yeah it is a TON of iterations

21:59 <tanelso2> Wasn't sure if there were more efficient ways to write this that handles the iterations better.

21:59 <tanelso2> Maybe a data structure similar to Clojure's transients? Not familiar enough with OCaml yet

22:00 <tanelso2> Wait nevermind

22:01 <tanelso2> Set.mem took the time down from 45 seconds to 0.08s

22:01 <tanelso2> Awesome. Thanks for the help everyone!

22:01 <tanelso2> I will probably be back with more questions in the coming weeks

22:07 <companion_cube> no pb, good luck tanelso2!

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