I’ll make a PR with count_until then, and we can see how it looks in code!
On Fri, Dec 4, 2020 at 2:29 AM José Valim <jose.va...@dashbit.co> wrote: > We can also add Enum.count_until(enumerable, filter, n) and you can use > filter = & &1 if you want to force enumeration, like there is for > Enum.count/2 today. > > On Fri, Dec 4, 2020 at 8:28 AM José Valim <jose.va...@dashbit.co> wrote: > >> 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. Any thoughts? >>>>> > >> >>>>> > >> -- >>>>> > >> You received this message because you are subscribed to the Google >>>>> > >> Groups "elixir-lang-core" group. >>>>> > >> To unsubscribe from this group and stop receiving emails from it, >>>>> > >> send an email to elixir-lang-core+unsubscr...@googlegroups.com. >>>>> > >> To view this discussion on the web visit >>>>> > >> >>>>> https://groups.google.com/d/msgid/elixir-lang-core/263d7c39-a32b-4294-93d8-40f248c9b3c8n%40googlegroups.com >>>>> > >> < >>>>> https://groups.google.com/d/msgid/elixir-lang-core/263d7c39-a32b-4294-93d8-40f248c9b3c8n%40googlegroups.com?utm_medium=email&utm_source=footer >>>>> > >>>>> > >> . >>>>> > >> >>>>> > >> -- >>>>> > >> You received this message because you are subscribed to the Google >>>>> > >> Groups "elixir-lang-core" group. >>>>> > >> To unsubscribe from this group and stop receiving emails from it, >>>>> > >> send an email to elixir-lang-core+unsubscr...@googlegroups.com. >>>>> > >> To view this discussion on the web visit >>>>> > >> >>>>> https://groups.google.com/d/msgid/elixir-lang-core/CAGnRm4JX4NE1yWH1G5L_DjF18v8zejF0%2BSkb_oz%3DPiUHM8Mz1w%40mail.gmail.com >>>>> > >> < >>>>> https://groups.google.com/d/msgid/elixir-lang-core/CAGnRm4JX4NE1yWH1G5L_DjF18v8zejF0%2BSkb_oz%3DPiUHM8Mz1w%40mail.gmail.com?utm_medium=email&utm_source=footer >>>>> > >>>>> > >> . >>>>> > >> >>>>> > >> -- >>>>> > >> You received this message because you are subscribed to the Google >>>>> > >> Groups "elixir-lang-core" group. >>>>> > >> To unsubscribe from this group and stop receiving emails from it, >>>>> > >> send an email to elixir-lang-core+unsubscr...@googlegroups.com. >>>>> > >> >>>>> > >> To view this discussion on the web visit >>>>> > >> >>>>> https://groups.google.com/d/msgid/elixir-lang-core/CAK-yb0BBGCrgbZamFs%2BeqLUis6mFQgvUHkKK1htSN5rDDWwMRQ%40mail.gmail.com >>>>> > >> < >>>>> https://groups.google.com/d/msgid/elixir-lang-core/CAK-yb0BBGCrgbZamFs%2BeqLUis6mFQgvUHkKK1htSN5rDDWwMRQ%40mail.gmail.com?utm_medium=email&utm_source=footer >>>>> > >>>>> > >> . >>>>> > >> >>>>> > >> -- >>>>> > >> You received this message because you are subscribed to the Google >>>>> > >> Groups "elixir-lang-core" group. >>>>> > >> To unsubscribe from this group and stop receiving emails from it, >>>>> > >> send an email to elixir-lang-core+unsubscr...@googlegroups.com. >>>>> > >> To view this discussion on the web visit >>>>> > >> >>>>> https://groups.google.com/d/msgid/elixir-lang-core/DB7PR07MB3899C92933992464F17898E1FAF20%40DB7PR07MB3899.eurprd07.prod.outlook.com >>>>> > >> < >>>>> https://groups.google.com/d/msgid/elixir-lang-core/DB7PR07MB3899C92933992464F17898E1FAF20%40DB7PR07MB3899.eurprd07.prod.outlook.com?utm_medium=email&utm_source=footer >>>>> > >>>>> > >> . >>>>> > >> >>>>> > > -- >>>>> > > You received this message because you are subscribed to the Google >>>>> > > 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