Thank you for the explanation. Is it possible to explain with an example? " sorter will apply to each element in the tuple one-by-one. But the same sorter applies to all elements in the tuple." I think I am not able to understand this part.
grangerelixx
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APB9785
Creator of ECSx
Let’s say the mapper returns two elements in a tuple:
&{Map.fetch!(&1, :post_body_size), Map.fetch!(&1, :date)}
like
{4, ~D[2021-06-05]}
{4, ~D[2021-06-04]}
{5, ~D[2021-06-02]}
etc...
Now if the sorter is &>=/2 (or :desc) then it will compare
4 >= 4
4 >= 5
etc...
But it will also use the same sorter &>=/2 to compare the second elements in the tuple:
~D[2021-06-05] >= ~D[2021-06-04]
~D[2021-06-04] >= ~D[2021-06-02]
And this does not work because you cannot use >=/2 to sort Structs (see Enum.sort/2 - Sorting structs). Instead, you must use the Date.compare function as the sorter by passing the Date module as the third argument:
post_list
|> Enum.sort_by(&{Map.fetch!(&1, :post_body_size), Map.fetch!(&1, :date)}, Date)
This is ok:
Date.compare(~D[2021-06-05], ~D[2021-06-04])
Date.compare(~D[2021-06-04], ~D[2021-06-02])
But now we have the opposite problem when sorting by :post_body_size:
Date.compare(4, 4)
Date.compare(4, 5)
This will not work because Date.compare expects %Date{} structs, not Integers.
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grangerelixx
Makes sense. Can we not add a tuple of sorters? We do something like this here Enum.sort_by(&Map.fetch!(&1, :post_body_size), :desc) so I tried {{:desc, Date}, :desc} but this didn’t work. Is it because of the arity issue? elixir/lib/elixir/lib/enum.ex at d7f96d833235fa471a58d3cfc22c01ca38e418bf · elixir-lang/elixir · GitHub
0
APB9785
Creator of ECSx
No. The possible options are:
(element(), element() -> boolean())
| :asc
| :desc
| module()
| {:asc | :desc, module()}
)
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