A handful of (mostly) arrow macros with superpowers.


Arrows #

A handful of (mostly) arrow macros.

Installation #

The package can be installed by adding arrows to your list of dependencies in mix.exs: -->

def deps do
    {:arrows, "~> 0.1.0"}

Or via git:

def deps do
    {:arrows, git: "", branch: "main"}

Documentation #

The Elixir |> ("pipe") operator is one of the things that seems to get people excited about elixir. Probably in part because you then don't have to keep coming up with function names. Unfortunately it's kind of limiting.
The moment you need to pipe a parameter into a position that isn't the first one, it breaks down and you have to drop out of the pipeline format or write a secondary function to handle it.

Not any more! By simply inserting ... where you would like the value to be inserted, Arrows will override where it is placed. This allows you to keep on piping while accommodating that function with the annoying argument order. Arrows was inspired by an existing library. Here is part of the test suite in lieu of examples:

defmodule ArrowsTest do
  use ExUnit.Case
  use Arrows

  def double(x), do: x * 2
  def double_fst(x, _), do: x * 2
  def double_snd(_, x), do: x * 2
  def add_snd_thd(_, x, y), do: x + y

  test "|>" do
    assert 4 == (2 |> double)
    assert 4 == (2 |> double())
    assert 4 == (2 |> double(...))
    assert 8 == (2 |> double(double(...)))
    assert 4 == (2 |> double_fst(1))
    assert 4 == (2 |> double_fst(..., 1))
    assert 8 == (2 |> double_fst(double(...), 1))
    assert 4 == (2 |> double_snd(1, ...))
    assert 8 == (2 |> double_snd(1, double(...)))
    assert 3 == (2 |> add_snd_thd(1, ..., 1))
    assert 4 == (2 |> add_snd_thd(1, ..., ...))
    assert 6 == (2 |> add_snd_thd(1, ..., double(...)))
    for x <- [:yes, 2, nil, false] do
      assert {:ok, x} == (x |> {:ok, ...})

A few little extra features you might notice here:

  • You can move the parameter into a subexpression, as in 2 |> double_fst(double(...), 1) where
    double will be called before the parameter is passed to double_fst.
  • You can use ... multiple times, substituting it in multiple places.
  • The right hand side need not even be a function call, you can use any expression with ....

Ok-pipe #

Arrows also provides an ok-pipe operator, ~>, which only pipes into the next function if the result from the last one was considered a success. It's inspired by OK, but we have chosen to do things slightly differently so it better fits with our regular pipe.

input result
{:ok, x} fun.(x)
{:error, e} {:error, e}
nil nil
x when not is_nil(x) fun.(x)

In the case of a function returning an ok/error tuple being on the left hand side, this is straightforward to determine. In the event of {:ok, x}, x will be passed into the right hand side to call. In the event of {:error, x}, the result will be {:error, x}.

We also deal with a lot of functions that indicate failure by returning nil. ~> tries to 'do what I mean' for both of these so you can have one pipe operator to rule them all. If nil is a valid result, you must thus be sure to wrap it in an ok tuple when it occurs on the left hand side of ~>.

|> and ~> compose in the way you'd expect; i.e. a ~> receiving an error tuple or nil will stop executing the rest of the chain of (mixed) pipes.

Documentation can be found at

Updated 25 Feb 2024

Bonfire Networks

Open Collective account Open Source