Is it possible to choose where the pipe output is inserted into Elixir function args?

Question

Consider a (smelly, non-idiomatic) function like the below:

def update_2d(array, inds, val) do
    [first_coord, second_coord] = inds

    new_arr = List.update_at(array, second_coord, fn(y) ->
      List.update_at(Enum.at(array, second_coord),
        first_coord, fn(x) -> val end) end)
end

This function will take in a list of lists, a list of two indices, and a value to insert within the list of lists at the location specified by the indices.

As a first step to making this more Elixir-ey, I start laying pipe:

array 
  |> Enum.at(second_coord) 
  |> List.update_at(first_coord, fn(x) -> val end)

That gets me most of the way there, but how do I pipe that output into the anonymous function of the last List.update_at call? I can nest it within the original call, but that seems like giving up:

List.update_at(array, second_coord, fn(y) -> 
  array 
  |> Enum.at(second_coord) 
  |> List.update_at(first_coord, fn(x) -> val end) 
end)

Gazler Gazler · Accepted Answer · 2015-08-23T10:24:08

You can simply bind to a variable to capture your first result and then replace it in the second List.update_at/3 call

def update_2d(array, inds = [first_coord, second_coord], val) do
  updated =
    array
    |> Enum.at(second_coord) 
    |> List.update_at(first_coord, fn(x) -> val end)

    List.update_at(array, second_coord, fn(x) -> updated end)
end

You can also use the capture operator to do this:

  def update_2d(array, inds = [first_coord, second_coord], val), do:    
    array
    |> Enum.at(second_coord) 
    |> List.update_at(first_coord, fn(x) -> val end)
    |> (&(List.update_at(array, second_coord, fn(y) -> &1 end))).()

I find using a variable much more readable, but the option is there.

Is it possible to choose where the pipe output is inserted into Elixir function args?

2 Answers