Pipe Operator (|>)
The pipe operator (|>) takes result of expression on left and inserts it as first parameter of function call on the right.
Instead of
defmodule URLWorker do
def start(url) do
do_request(HTTPoison.get(url))
end
# ...
end
Use pipe operators. The more you use it, the more you’ll see data as being transformed from one form to another, like an assembly line.
defmodule URLWorker do
def start(url) do
result = url |> HTTPoison.get |> do_request
end
# ...
end
Ex: Filtering files in directory by filename
Let’s say you have a directory filled with e-books. This directory may have nested folders. You want to get filenames of only the Java-related EPUBs. You only want books with filenames that end with *.epub and include Java:
It’s nice to read code in which steps are so explicit and obvious.
Erlang interoperability
Since both Elixir and Erlang share the same bytecode, calling Erlang code doesn’t affect performance in any way. We may call any Erlang library with Elixir code!
Erlang's :random.uniform/1 function:
Make random number in Erlang:
> random:uniform(123).
55
Use same Erlang function in Elixir:
> :random.uniform(123)
55
Note positions of colon and dot. Those are the only differences between Erlang + Elixir syntax for Erlang functions!
We may see documentation for Erlang functions in iex:
> h :random.uniform/1
uniform/1
@spec uniform(n) :: pos_integer() when n: pos_integer()
deprecated: random:uniform/1 is deprecated; use the 'rand' module instead
Returns, for a specified integer N >= 1, a random integer uniformly distributed
between 1 and N, updating the state in the process dictionary.
Calling Erlang HTTP client in Elixir
When Elixir lacks a feature, check if Erlang has a standard library function we can use before searching for third-party libraries.
To download a web page, we need an HTTP client. Elixir doesn’t come with a built-in HTTP client.It doesn’t need to, since Erlang comes with :httpc.request/1. Also see :httpc cheatsheet.
First start the inets application (it’s in the Erlang doc), then make the HTTP request:
> :inets.start
:ok
> {:ok, {status, headers, body}} = :httpc.request 'http://www.elixir-lang.org'
{:ok,
{{~c"HTTP/1.1", 200, ~c"OK"},
[
{~c"cache-control", ~c"max-age=600"},
{~c"connection", ~c"keep-alive"},
{~c"date", ~c"Sun, 24 Dec 2023 16:53:03 GMT"},
{~c"via", ~c"1.1 varnish"},
...
> status
{~c"HTTP/1.1", 200, ~c"OK"}
> headers
[
{~c"cache-control", ~c"max-age=600"},
{~c"connection", ~c"keep-alive"},
{~c"date", ~c"Sun, 24 Dec 2023 16:53:53 GMT"},
{~c"via", ~c"1.1 varnish"},
{~c"accept-ranges", ~c"bytes"},
...
> body
~c"<!DOCTYPE html>\n<html xmlns=\"http://www.w3.org/1999/xhtml\" lang=\"en\">\n<head>\n <meta http-equiv=\"Content-Type\" content=\"text/html; charset=UTF-8\" />\n <meta http-equiv=\"X-UA-Compatible\" content=\"IE=edge\" />\n <meta name=\"description\" content=\"Welcome to Elixir, a dynamic, functional language designed for building scalable and maintainable applications\">\n <title>The Elixir programming language</title>\n
...
Inspect Erlang Virtual Machine with Observer
Erlang Observer is a GUI front end that lets us inspect the Erlang virtual machine, among other things:
See its Docs. 47-page PDF.
To run:
In Elixir:
> :observer.start
In Erlang (from Bash shell):
> erl -sname observer -hidden -setcookie MyCookie -run observer
Observer lets us see how much load the VM is taking and the layout of our supervision trees (more in chapter 6). We also see data stored in the built-in Erlang database(s), memory usage, which processes are running, etc.
