It's great that there exists a total order (structural) in Elixir/Erlang, I
just wish it wasn't accessible with `<`, `>`, as it is too error prone and
is simply never what one wants to do (at least in our app). Elixir 2.0? 😆
At work I just recently overloaded them to raise unless both arguments are
`is_number`, and we found bugs where we were comparing Decimals, and other
bugs where we were comparing with `nil`. They are no longer allowed in
guards too.
On Friday, 3 March 2023 at 09:31:28 UTC-5 william.l...@cargosense.com wrote:
> > if I’m remembering `DateTime.compare/2` correctly
>
> Close! The `Module.compare/2` functions return one of `:lt`, `:eq`, or
> `:gt` ("less than", "equal to", "greater than"), similar to what Haskell
> does. You may have been thinking of something like OCaml where `compare`
> returns `-1`, `0`, or `1` resp.
>
> > So Why don't we implicitly sort it so that it can be compared by
> inequality sign(> or <)?
>
> To clarify, functions like `<` *define* the sort order.
>
> Any time you sort a list, you're using a function that compares two
> elements. Even if you call `Enum.sort/1`, you're implicitly using `<=/2` as
> the comparison function. If you want some other sort order, e.g. for
> semantic ordering of `DateTime`s, then you must supply your own comparison
> function.
>
> The reason that you can use `<` on structs with `CompareChain` is that it
> uses macros to re-write an expression like
>
> `~D[2023-03-03] < ~D[2023-03-04]`
>
> as
>
> `Date.compare(~D[2023-03-03], ~D[2023-03-04]) == :lt`.
>
> But that doesn't change the behavior of `<` itself. We're basically stuck
> with what `<` and the like do. Though as José points out, that's actually a
> good thing.
>
> (Side note, you actually have to call `compare?(~D[2023-03-03] <
> ~D[2023-03-04], Date)` with `CompareChain` to invoke the re-write. I just
> wanted the example to be more readable.)
> On Friday, March 3, 2023 at 3:27:00 AM UTC-5 José Valim wrote:
>
>> It is also important to note that both kinds of comparisons are important
>> to have in a language. The docs for main discuss this:
>> https://hexdocs.pm/elixir/main/Kernel.html#module-structural-comparison
>>
>> On Fri, Mar 3, 2023 at 7:47 AM Austin Ziegler <halos...@gmail.com> wrote:
>>
>>> In this case, because Elixir is passing the `<` and `>` comparisons to
>>> the underlying BEAM operations and there’s no overloading to say that `left
>>> < right` should mean `DateTime.compare(left, right) < 0` and `left > right`
>>> should mean `DateTime.compare(left, right) > 0` (if I’m remembering
>>> `DateTime.compare/2` correctly).
>>>
>>> `CompareChain` does that, but it’s something that gets opted into.
>>>
>>> -a
>>>
>>> On Thu, Mar 2, 2023 at 10:42 PM 최병욱 <cbw...@gmail.com> wrote:
>>>
>>>> So Why don't we implicitly sort it so that it can be compared by
>>>> inequality sign(> or <)?
>>>>
>>>> 2023년 3월 3일 금요일 오전 10시 3분 25초 UTC+9에 william.l...@cargosense.com님이 작성:
>>>>
>>>>> Shameless plug: I wrote a library called `CompareChain` that allows
>>>>> you to use operators like `<` and `>` on structs like `DateTime`.
>>>>>
>>>>> Hexdocs: https://hexdocs.pm/compare_chain/readme.html
>>>>>
>>>>> On Thursday, March 2, 2023 at 10:54:08 AM UTC-5 Jay Rogov wrote:
>>>>>
>>>>>> Because the underlying structure used to represent DateTime is a
>>>>>> struct, which is simply a map under the hood.
>>>>>> Erlang/Elixir uses a rather arbitrary order of keys (e.g. hour ->
>>>>>> year -> day -> minute) when comparing 2 maps which you can't control.
>>>>>>
>>>>>> Thus, you need to have a specific function that would compare these
>>>>>> structs according to implied field order (year -> month -> day -> hour
>>>>>> ->
>>>>>> etc.)
>>>>>>
>>>>>> More:
>>>>>> https://hexdocs.pm/elixir/main/NaiveDateTime.html#module-comparing-naive-date-times
>>>>>>
>>>>>> On Thursday, 2 March 2023 at 4:38:00 pm UTC+1 cbw...@gmail.com wrote:
>>>>>>
>>>>>>> Can't you compare DateTime with '>' or '<' instead of
>>>>>>> DateTime.compare?
>>>>>>>
>>>>>> --
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>>>> <https://groups.google.com/d/msgid/elixir-lang-core/afa3830a-8944-4e12-84cc-d8e28d9fceb0n%40googlegroups.com?utm_medium=email&utm_source=footer>
>>>> .
>>>>
>>>
>>>
>>> --
>>> Austin Ziegler • halos...@gmail.com • aus...@halostatue.ca
>>> http://www.halostatue.ca/ • http://twitter.com/halostatue
>>>
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>>>
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>>>
>>
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