IEx

Mix.install([
  {:jason, "~> 1.4"},
  {:kino, "~> 0.9", override: true},
  {:youtube, github: "brooklinjazz/youtube"},
  {:hidden_cell, github: "brooklinjazz/hidden_cell"}
])

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IEx

IEx is Elixir’s interactive shell. If you install Elixir on your computer, you can run iex in your command line to start the interactive shell. It will print information about your IEx environment, such as what Elixir and Erlang/OTP version you have installed.

$ iex
Erlang/OTP 24 [erts-12.3] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit]

Interactive Elixir (1.13.3) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)>

We can evaluate Elixir code inside of the IEx shell. IEx is often useful for small experiments, running Elixir scripts, and even as a playground environment building/running Elixir applications.

Your Turn

Execute some simple arithmetic in the iex shell, like so.

iex > 4 + 4
8

IO

We can use the IO module for retrieving user input and displaying output. We’ll learn more about modules in later lessons.

Wrap any value in IO.inspect() to print it.

IO.inspect(2 + 2)

We can also use IO.gets/2 to retrieve input from the user.

iex> input = IO.gets("Give me some input: ")
Give me some input: sure!
"sure!\n"
iex> input
"sure!\n"

Keep in mind, this will work when running an Elixir project or in the IEx shell, but will fail with an :enotsup (error not supported) error when we run it from Livebook.

IO.gets("I will fail!")

That’s because in Livebook there is no command line or other device to retrieve the user input from.

Your Turn

Run IO.gets/2 in the command line from the IEx shell and bind the return value to an input variable.

iex> input = IO.gets("Give me some input: ")

Multiline Expressions

You can break your Elixir code into multiple lines in the IEx shell, which only evaluates when the expression is complete.

Your Turn

Write 4 + in the IEx shell, then press enter to break the expression into multiple lines. Then finish of the expression by writing another integer such as 5.

iex >
  4 +
    ... > 5

9

Your Turn

Create a module Greeting in the IEx shell, then call the hello/0 function.

iex> defmodule Greeting do
iex>  def hello do
iex>   "Hello, world!"
iex>  end
iex> end
{:module, Greeting,
 <<70, 79, 82, 49, 0, 0, 4, 204, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 0, 140,
   0, 0, 0, 14, 15, 69, 108, 105, 120, 105, 114, 46, 71, 114, 101, 101, 116,
   105, 110, 103, 8, 95, 95, 105, 110, 102, 111, ...>>, {:hello, 0}}
iex> Greeting.hello()
"Hello, world!"

We’ll learn more about modules in future lessons.

Closing The IEx Shell

Once we’ve started the IEx shell, you can close it by pressing CTRL+\.

Your Turn

First, define a variable in your current IEx shell.

iex> hello = "world"

Close the current IEx shell, then reopen it with iex command. Notice that previously defined variables from the previous IEx shell do not exist.

iex(1)> hello
** (CompileError) iex:1: undefined function hello/0 (there is no such import)

Autocomplete

Much like in livebook, you can autocomplete Elixir expressions by pressing Tab.

iex> Enum.a
all?/1    all?/2    any?/1    any?/2    at/2      at/3

IEx Helpers

The built-in IEx.Helpers module provides helpers to make it easier to work with the IEx shell.

Help (h)

The h helper prints the documentation for any code.

For example, execute the following in your IEx shell to view the Enum documentation.

iex> h Enum

Information (i)

The i helper provides information about any Elixir term (data type).

For example, execute the following in your IEx shell to view more information about the Integer data type.

iex> i 2
Term
  2
Data type
  Integer
Reference modules
  Integer
Implemented protocols
  IEx.Info, Inspect, List.Chars, String.Chars

Compile (c)

The c helper can load an Elixir file into your IEx shell.

For example, create a hello.exs file with your command line. Add the following contents to the file with your code editor.

defmodule Greeting do
  def hello do
    "Hello, world!"
  end
end

IO.puts("Executing hello.exs")

Then start an IEx session and load the file. Notice that IO.puts("Executing hello.exs") runs immediately.

iex> c "hello.exs"
Executing hello.exs
[Greeting]

The Greeting module is now available in the current IEx session.

iex> Greeting.hello()
"Hello, world!"

The iex command also accepts a filename as an argument to load a file into the IEx shell.

$ iex hello.exs
iex> Greeting.hello()

Recompile (r)

The r helper recompiles a module in the current IEx shell.

For example, change the Greeting module in hello.exs to return a different value.

defmodule Greeting do
  def hello do
    "Hello!" # changed value
  end
end

IO.puts("Executing hello.exs")

Now, recompile the module and execute the hello/0 function.

iex> r Greeting
iex> Greeting.hello()
"Hello!"

Value (v)

The v helper returns the value on a given line.

We’ve omitted the line numbers in the IEx shell in previous examples, but you’ll notice each line has a corresponding number.

iex(1)> 1 + 1
2

To use the return value from a previous expression, use v/1 with the desired line number.

iex(2)> v(1) + 2
4

Alternatively, we can pipe (|>) the previous return value into the current expression.

iex(1)> %{}
iex(2)> |> Map.put(:key, "value")
%{key: "value"}

clear

if the IEx shell is getting messy, we can use the clear helper to clear the console screen.

iex> clear

Further Reading

IEx is generally helpful for experimenting with Elixir code and exploring an existing project or script. For more, consider reading the following:

• HexDocs: IEx.Helpers includes a full list of helper functions and their usage beyond those covered above.
• Elixir Schools: IEx.Helpers an Elixir schools lesson on IEx.Helpers.

Commit Your Progress

DockYard Academy now recommends you use the latest Release rather than forking or cloning our repository.

Run git status to ensure there are no undesirable changes. Then run the following in your command line from the curriculum folder to commit your progress.

$ git add .
$ git commit -m "finish IEx reading"
$ git push

We’re proud to offer our open-source curriculum free of charge for anyone to learn from at their own pace.

We also offer a paid course where you can learn from an instructor alongside a cohort of your peers. We will accept applications for the June-August 2023 cohort soon.

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