Elixir - Enum and Dates
Properly sorting & min / max dates without passing a type created unexpected results
Context
Recently we had a little bug in our code we were looking for a date-range (minimum begin-date and maximum end-date). We realized Enum.sort, Enum.min, Enum.max, Enum.sort_by, Enum.min_by, Enum.max_by are not reliable with dates.
for example the following causes problems - as you can see different dates will sort in unexpected ways and differently depending upon the dates:
dates_reverse = [~D[2023-03-28],~D[2024-05-18],~D[2025-07-08]]
dates_unclear = [~D[2017-02-28],~D[2019-02-01],~D[2022-05-01]]
Enum.sort(dates_reverse)
[~D[2025-07-08], ~D[2024-05-18], ~D[2023-03-28]]
Enum.sort(dates_unclear)
[~D[2019-02-01], ~D[2022-05-01], ~D[2017-02-28]]
Enum.min(dates_reverse)
~D[2025-07-08]
Enum.min(dates_unclear)
~D[2019-02-01]
Enum.max(dates_reverse)
~D[2023-03-28]
Enum.max(dates_unclear)
~D[2017-02-28]
However, as of Elixir 1.10 you can pass a type Date parameter, thus the following works reliably as expected:
dates_reverse = [~D[2023-03-28],~D[2024-05-18],~D[2025-07-08]]
dates_unclear = [~D[2017-02-28],~D[2019-02-01],~D[2022-05-01]]
dates_reverse |> Enum.sort(Date)
[~D[2023-03-28], ~D[2024-05-18], ~D[2025-07-08]]
dates_unclear |> Enum.sort(Date)
[~D[2017-02-28], ~D[2019-02-01], ~D[2022-05-01]]
dates_reverse |> Enum.min(Date)
~D[2023-03-28]
dates_unclear |> Enum.min(Date)
~D[2017-02-28]
dates_reverse |> Enum.max(Date)
~D[2025-07-08]
dates_unclear |> Enum.max(Date)
~D[2022-05-01]
Resources
Bill Tihen
Developer, Data Enthusiast, Educator and Nature’s Friend
very curious – known to explore knownledge and nature