Generate a list of dates

Question

Let's say I'm using Timex as follows:

use Timex
Interval.new(from: ~D[2016-03-03], until: [days: 3])
%Elixir.Timex.Interval{from: ~N[2016-03-03 00:00:00], left_open: false, right_open: true, step: [days: 1], until: ~N[2016-03-06 00:00:00]}

I want to generate a list of dates, one day apart. How do I go from this to a list?

Answer 1

Why would you use 3rd party libraries like Timex for such a simple thing?

Enum.map(0..3, &Date.add(~D[2016-03-03], &1))
#⇒ [~D[2016-03-03], ~D[2016-03-04], ~D[2016-03-05], ~D[2016-03-06]]

Answer 2

How do I go from [an Interval] to a list?

In the source code for Timex.Interval, there are some examples of what you can do. Here's a modified version of one of those examples:

  def days_as_strings(interval) do
    interval
    |> Interval.with_step([days: 1]) 
    |> Enum.map(&Timex.format!(&1, "%Y-%m-%d", :strftime))
  end

In the shell:

~/elixir_programs/http$ iex -S mix
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Interactive Elixir (1.6.6) - press Ctrl+C to exit (type h() ENTER for help)

iex(1)> use Timex
Timex.Timezone

iex(2)> interval = Interval.new(from: ~D[2016-03-03], until: [days: 3])
%Timex.Interval{
  from: ~N[2016-03-03 00:00:00],
  left_open: false,
  right_open: true,
  step: [days: 1],
  until: ~N[2016-03-06 00:00:00]
}

iex(3)> MyTime.days_as_strings(interval)
["2016-03-03", "2016-03-04", "2016-03-05"]

iex(4)> 

Notice that Enum.map() is called as the last step in the pipeline, so whatever the previous line returned is an Enumerable. Well, the Enum module also defines the function Enum.to_list(), so let's try that instead of Enum.map():

  def days_as_dates(interval) do
    interval
    |> Interval.with_step([days: 1])
    |> Enum.to_list()
  end

In the shell:

~/elixir_programs/http$ iex -S mix
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Interactive Elixir (1.6.6) - press Ctrl+C to exit (type h() ENTER for help)

iex(1)> use Timex
Timex.Timezone

iex(2)> interval = Interval.new(from: ~D[2016-03-03], until: [days: 3])
%Timex.Interval{
  from: ~N[2016-03-03 00:00:00],
  left_open: false,
  right_open: true,
  step: [days: 1],
  until: ~N[2016-03-06 00:00:00]
}

iex(3)> MyTime.days_as_dates(interval)
[~N[2016-03-03 00:00:00], ~N[2016-03-04 00:00:00],
 ~N[2016-03-05 00:00:00]]

iex(4)> 

The output shows that the elements of the Enumerable are NaiveDateTime's, which are datetime's with no timezone info.

The NaiveDateTime module defines a function to_date/1, so we can map to_date/1 onto the Interval's elements to get Date's:

  def days_as_dates(interval) do
    interval
    |> Interval.with_step([days: 1])
    |> Enum.map(&NaiveDateTime.to_date/1)
  end

In the shell:

~/elixir_programs/http$ iex -S mix
Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Compiling 1 file (.ex)
Interactive Elixir (1.6.6) - press Ctrl+C to exit (type h() ENTER for help)

iex(1)> use Timex
Timex.Timezone 

iex(2)> interval = Interval.new(from: ~D[2016-03-03], until: [days: 3])
%Timex.Interval{
  from: ~N[2016-03-03 00:00:00],
  left_open: false,
  right_open: true,
  step: [days: 1],
  until: ~N[2016-03-06 00:00:00]
}

iex(3)> MyTime.days_as_dates(interval)                    
[~D[2016-03-03], ~D[2016-03-04], ~D[2016-03-05]]

iex(4)> 

And, it turns out that the default step is [days: 1] when you iterate over the Interval, so you can just write:

  def days_as_dates(interval) do
    for ndt <- interval, do: NaiveDateTime.to_date(ndt)
  end