That's a very good point Jayson. I think we should go with "count until should take advantage of all optimizations and ignore side-effects”. I believe it is fair to expect that no enumerable that implements count actually has side-effects, exactly because of the implications of what you said.
On Fri, Dec 4, 2020 at 8:08 AM 'Jayson Vantuyl' via elixir-lang-core < elixir-lang-core@googlegroups.com> wrote: > There are three questions I don’t think we’re considering: > * What does it mean to “partially count” an Enumerable that implements an > “efficient” `count/1` function? > * If such an Enumerable has side-effects for its `reduce/3` function, > should they be somehow still happen even though the `count/1` doesn’t > necessarily iterate the elements? > * If such an Enumerable returns a larger count that asked for, should we > return the larger “technically correct” value; or the `max + 1` value? > > I generally like `count_until/2` because it‘s unopinionated about what > you’re doing with the count. But the answers to the above question probably > should be addressed and documented. > > I really see two ways to address the above question. Either we consider > “count until implies actively counting” or “count until should take > advantage of all optimizations and ignore side-effects”. > > My feel is that the latter is generally going to be more efficient in the > common case but the former is less likely to create unexpected behavior > from people who don’t know how their Enumerable is implemented. > > I’m inclined to favor the former. It won’t throw away efficiency that a > custom Enumerable will implement, it’ll generally make naive code faster, > and the rare cases where people expect side-effects is probably less > important than either of those other benefits. > > Sent from my iPhone > > On Dec 3, 2020, at 21:18, José Valim <jose.va...@dashbit.co> wrote: > > > Thanks Allen! I believe that's a good idea. > > I think the main insight is that we don't want a predicate function > (at_least? more_than?). Using compare returns three states - which is > better than two - but what if we just returned the number? After all, if I > am interested in knowing if something has less than 10, 10, or more than > 10, I just need to count until eleven. Returning a number seems to be more > flexible too. Therefore, what do you think about: count_until(enum, value)? > > To check if less, eq, or more than 10: > > case Enum.count_until(count, 10 + 1) do > 11 -> :gt > 10 -> :eq > _ -> :lt > end > > For at least 10: > > Enum.count_until(count, 10) == 10 > > For more than 10: > > Enum.count_until(count, 10 + 1) > 10 > > Thoughts? > > > On Fri, Dec 4, 2020 at 2:14 AM Zach Daniel <zachary.s.dan...@gmail.com> > wrote: > >> Yep! I really like it :) >> >> On Thu, Dec 3, 2020 at 7:52 PM eksperimental <eksperimen...@autistici.org> >> wrote: >> >>> On Thu, 3 Dec 2020 19:06:18 -0500 >>> Allen Madsen <allen.c.mad...@gmail.com> wrote: >>> >>> > Enum.compare_count([], 1) #=> :lt >>> > Enum.compare_count([1], 1) #=> :eq >>> > Enum.compare_count([1, 2], 1) #=> :gt >>> >>> This is the way to go, because in one function call we can determine the >>> course of the action, such as in >>> >>> case Enum.compare_count(list, n) do >>> :lt -> ... >>> :eq -> ... >>> :gt -> ... >>> end >>> >>> when using the predicate functions it would require at least two >>> function calls. >>> >>> > >>> > Allen Madsen >>> > http://www.allenmadsen.com >>> > >>> > >>> > On Thu, Dec 3, 2020 at 6:51 PM Zach Daniel >>> > <zachary.s.dan...@gmail.com> wrote: >>> > >>> > > Well, List.count doesn’t exist yet, but either way it sounds like >>> > > not a great idea :) I couldn’t find examples in other Lang’s, so >>> > > maybe I’ll just throw out some other names: >>> > > >>> > > Enum.at_least?/2 >>> > > >>> > > Enum.at_most?/2 >>> > > >>> > > Enum.has_count?/2 >>> > > >>> > > On Thu, Dec 3, 2020 at 5:14 PM Michał Muskała <mic...@muskala.eu> >>> > > wrote: >>> > > >>> > >> Unfortunately this can’t be done automatically since it has subtle >>> > >> semantic differences. In particular Enum.count/1 (or length/1) not >>> > >> only traverses the list to count its size, but also verifies it’s >>> > >> a proper list raising an exception for improper lists. The >>> > >> difference could be seen for value like: >>> > >> >>> > >> >>> > >> >>> > >> [1, 2, 3 | :invalid] >>> > >> >>> > >> >>> > >> >>> > >> Calling length/1 or Enum.count/1 on this raises. If compiler did >>> > >> the optimisation you propose, for something like length(list) > 0, >>> > >> it wouldn’t fully traverse the list and wouldn’t raise. Thus such >>> > >> an optimisation is not possible in the general case. >>> > >> >>> > >> >>> > >> >>> > >> *From: *elixir-lang-core@googlegroups.com < >>> > >> elixir-lang-core@googlegroups.com> >>> > >> *Date: *Thursday, 3 December 2020 at 22:04 >>> > >> *To: *elixir-lang-core@googlegroups.com < >>> > >> elixir-lang-core@googlegroups.com> >>> > >> *Subject: *Re: [elixir-core:9802] Proposal `Enum.more_than?/2` or >>> > >> `List.more_than?/2` >>> > >> >>> > >> This probably off the table/unreasonable, but it also seems like >>> > >> something that could be statically solved and people would never >>> > >> need to know as it is just an optimization. E.g Enum.count(list) > >>> > >> n could optimized by the compiler? Probably wouldn’t be good for >>> > >> all Enums, since counting would be expected to enumerate them, so >>> > >> maybe only something like List.count 🤷♂️ >>> > >> >>> > >> >>> > >> >>> > >> On Thu, Dec 3, 2020 at 1:42 PM Zach Daniel >>> > >> <zachary.s.dan...@gmail.com> wrote: >>> > >> >>> > >> Another benefit to the options list would be supporting it for >>> > >> count with a predicate, e.g Enum.count(enum, &some_predicate/1, >>> > >> max: 4) >>> > >> >>> > >> >>> > >> >>> > >> On Thu, Dec 3, 2020 at 1:35 PM Zach Daniel >>> > >> <zachary.s.dan...@gmail.com> wrote: >>> > >> >>> > >> Nothing is jumping out at me from elsewhere yet, but another >>> > >> option might be accepting options in `Enum.count`, like >>> > >> `Enum.count(list, max: 4)`. I’ll keep searching though. >>> > >> >>> > >> >>> > >> >>> > >> On Thu, Dec 3, 2020 at 1:31 PM Zach Daniel >>> > >> <zachary.s.dan...@gmail.com> wrote: >>> > >> >>> > >> I agree on the name feeling subpar :) I’ll take a look and see if >>> > >> I can find other examples. >>> > >> >>> > >> >>> > >> >>> > >> On Thu, Dec 3, 2020 at 12:21 PM José Valim <jose.va...@dashbit.co> >>> > >> wrote: >>> > >> >>> > >> Thanks Zach! I like this idea but the proposed name, for some >>> > >> reason, doesn't sit right with me. Is there any prior art from >>> > >> other langs we could look at? >>> > >> >>> > >> >>> > >> >>> > >> On Thu, Dec 3, 2020 at 6:15 PM Zachary Daniel >>> > >> <zachary.s.dan...@gmail.com> wrote: >>> > >> >>> > >> Counting a list, especially a large one, to know if there are >>> > >> "more than x" or "less than x" items is inefficient. >>> > >> >>> > >> >>> > >> >>> > >> Right now I often see things like `if Enum.count(list) > 4 ...`, >>> > >> mostly because writing a recursive `more_than?` check is tedious, >>> > >> or doing something like `Enum.empty?(Enum.drop(list, 4))` is not >>> > >> very expressive. >>> > >> >>> > >> >>> > >> >>> > >> I think it would be nice to have an `Enum.more_than?` that does >>> > >> that work for you. It could also be `List.more_than?/2` if we >>> > >> don't want it in Enum. 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