> 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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>>
>>
>> --
>> Austin Ziegler • halos...@gmail.com • aus...@halostatue.ca
>> http://www.halostatue.ca/ • http://twitter.com/halostatue
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