Omer Wazir

Learning Elixir - things you can do in IEx

For the Tucson Elixir meetup I gave a lightning talk on IEx, explaining some things that a new Elixir developer might not know.

If you don’t know what IEx is it’s the interactive elixir shell which is included when Elixir is installed on your computer. Since this post is based on a lightning talk I won’t cover every single feature but I will mention a few things that I think are good to know.

A few of the things IEx can do for you

All of the features in IEx are detailed here

Start IEx

Open your terminal and launch iex by simply typing iex and then enter. This interactive mode evaluates any Elixir expression which makes it an excellent place to get comfortable with Elixir & OTP.

$ iex
Erlang/OTP 19 [erts-8.2] [source] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false]

Interactive Elixir (1.4.1) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> person = %{name: "Remo Williams"}
%{name: "Remo Williams"}
iex(2)>

IEx.helpers

One of the most useful helpers is the h/1 function available from the IEx.Helpers module. To see everything that IEx.Helpers provides run h() inside IEx. To learn about IEx run h(IEx).

Some examples:

respawn() is another function which I use a lot to start a new shell process. Take the time to become familiar with the helpers available from IEx.Helpers.

Autocomplete

If you don't know what functions are available in a module, type the module name with a dot on the end and press tab to see a list of available functions. I have an example of using the autocomplete feature with the Map module, type Map. and make sure there is a dot after Map, then press tab and you should see a list of functions/arity pairs. Pretty neat!

iex(1)> Map.
delete/2             drop/2               equal?/2
fetch!/2             fetch/2              from_struct/1
get/2                get/3                get_and_update!/3
get_and_update/3     get_lazy/3           has_key?/2
keys/1               merge/2              merge/3
new/0                new/1                new/2
pop/2                pop/3                pop_lazy/3
put/3                put_new/3            put_new_lazy/3
split/2              take/2               to_list/1
update!/3            update/4             values/1

#iex:break

This is one of the best features.

In the example below a Map is not closed properly, leaving the expression open and creating chaos. Typing #iex:break will essentially break the expression and allow you to continue as if nothing happened. You will still need to type the expression in again and close it properly. This is probably my favorite feature and something I didn’t know until I started preparing for my talk.

iex(2)> %{user: "Artoo",
...(2)> name: "R2D2',
...(2)> type: 'robot'}
...(2)> .
...(2)> ;
...(2)> end
...(2)> %}
...(2)> #iex:break
** (TokenMissingError) iex:2: incomplete expression

Shell Configuration

Personally I have not configured my IEx shell and have left the default settings. It’s good to know what is configurable in case changes need to be made.

This is a list of what you can change as of Elixir 1.4:

:inspect

Inspect options used by the shell when printing results of expression evaluation. If you want to see everything during IO.inspect/1 then do this:

IEx.configure [inspect: [limit: :infinity]]

See more here Inspect.Opts.

Note: different kinds of inspects

inspect/1 and i/1 are different.

iex(2)> foo = %{ name: "Bruce Wayne", location: "Gotham"}
%{location: "Gotham", name: "Bruce Wayne"}
iex(3)> inspect(foo)
"%{location: \"Gotham\", name: \"Bruce Wayne\"}"
iex(4)> i(foo)
Term
  %{location: "Gotham", name: "Bruce Wayne"}
Data type
  Map
Reference modules
  Map
Implemented protocols
  IEx.Info, Collectable, Enumerable, Inspect

Pry into code

Debugging is entirely worthy of its own discussion but IEx.pry should be briefly mentioned here. You will need to require IEx in a module before you can use pry/0. Notice in the example below that require IEx and IEx.pry are both used together. pry/0 was new to me since I did not have a background in Ruby, it’s a lot like debugger; from JavaScript except when using pry “Setting variables or importing modules in IEx does not affect the caller the environment” (from the pry docs).

defmodule say do
 require IEx

 do something() do
  IEx.pry

  IO.puts("Hello, World")
 end
end

I hope you have learned a little bit more about what IEx offers to you, and you now know where to find out more. Things change with new releases so check with the latest documentation to know how a feature works. 😀