Part- 4

1. Running Multiple Mix Tasks

mix do deps.get,compile

You can run multiple tasks by separating them with coma ,

How ever you can also create aliases in your mix project in a file called mix.exs .

The project definition looks like the following way when you create one using a mix tool.

def project do
    [app: :project_name,
     version: "0.1.0",
     elixir: "~> 1.4-rc",
     build_embedded: Mix.env == :prod,
     start_permanent: Mix.env == :prod,
     deps: deps()]

You are also allowed to add some extra fields…

Here you have to add the aliases field.

 aliases: aliases()

Don’t forget to add , at the end when you add this in the middle of list .

The aliases() should return the key-value list.

defp aliases do
    "ecto.setup": ["ecto.create", "ecto.migrate", "ecto.seed"]

So, whenever you run the mix ecto.setup the three tasks ecto.create, ecto.migrate and ecto.seed will run one after the other.

You can also add them directly as following unlike I did with private function.

def project do
    [app: :project_name,
     version: "0.1.0",
     aliases: ["ecto.setup": ["ecto.create", "ecto.migrate", "ecto.seed"]]      

2. Accessing the Documentation

Elixir stores the documentation inside the bytecode in a memory. You access the documentation with the help of Code.get_docs/2 function . This means, the documentation accessed when it is required, but not when it is loaded in the virtual machine like iex

Suppose you defined a module in memory like ones you defined in IEx, cannot have their documentation accessed as they do not have their bytecode written to disk.

Let us check this…

Create a module with name test.ex with the following code. You can copy and paste it.

defmodule Test do
  @moduledoc """
  This is the test module docs

  @doc """
  This is the documentation of hello function
  def hello do
    IO.puts "hello"

Now stay in the directory where your file exists and run the command

$ iex test.ex

Now you can access the function definitions but not the documentation.

iex> Test.hello

That means the code is compiled but documentation is not stored in the memory. So, you cannot access the docs. Lets check that…

iex> Code.get_docs Hello, :moduledoc

You will see the output as nil when you are trying to access the docs of the module you have created so far. This is because, the bytecode is not available in disk. In simple way beam file is not present. Lets do that...

Press Ctrl+C twice so you will come out of the shell and this time you run the command as

$ elixirc test.ex

After running the command, you will see a file with name Elixir.Test.beam . Now the bytecode for the module Test is available in memory. Now you can access the documentation as follows...

$ iex
iex> Code.get_docs Test, :moduledoc
{3, "This is the test module docs\n"}

The output is tuple with two elements. The first element is the line number of the documentation it starts and second element is the actual documentation in the binary form.

You can read more about this function here

3. Verbose Testing

When you go with mix test it will run all the tests defined and gives you the time of testing. However, you can see more verbose output like which test you are running with the --trace option like following…

mix test --trace

It will list out the all tests with names you defined as test "test_string" here test_string is the name of the test.

4. Dynamic Function Name in Elixir Macro

defmacro gen_function(fun_name) do
  quote do 
    def unquote(:"#{fun_name}")() do
      # your code...

To be simple the name of the function should be an atom instead of binary.

5. Run Shell Commands in Elixir

System.cmd(command, args, options \\ [])

Executes the given command with args.

  • command is expected to be an executable available in PATH unless an absolute path is given.

  • args must be a list of binaries which the executable will receive as its

    arguments as is. This means that:


iex> System.cmd "echo", ["hello"]
    {"hello\n", 0}
iex> System.cmd "echo", ["hello"], into: []
    {["hello\n"], 0}

Get help from iex with h System.cmd

Checkout the documentation about System for more information and also check Erlang os Module.

6. Printing List as List without ASCII-Encoding

You know that when the list contains all the numbers as ASCII values, it will list out those values instead of the original numbers. Lets check that…

iex> IO.inspect [97, 98]

The code point of a is 97 and b is 98 hence it is listing out them as char_list. However you can tell the IO.inspect to list them as list itself with option char_lists: :as_list .

iex> IO.inspect [97, 98], charlists: :as_lists
[97, 98]

Open iex and type h Inspect.Opts, you will see that Elixir does this kind of thing with other values as well, specifically structs and binaries.

7. Accessing file name and line number etc…

defmacro __ENV__()

This macro gives the current environment information. You can get the information like current filename line function and others…

iex(4)> __ENV__.file

iex(5)> __ENV__.line

8. Creating Manual Pids

You can create the pid manually in Elixir with pid function. This comes with two flavors.

def pid(string)

Creates the pid from the string.

iex> pid("0.21.32")

def pid(a, b, c)

Creates a PID with 3 non negative integers passed as arguments to the function.

iex> pid(0, 21, 32)

Why do you create the pids manually?

Suppose you are writing a library and you want to test one of your functions for the type pid, then you can create one and test over it.

You cannot create the pid like assigning pid = #PID<0.21.32> because # is considered as comment here.

iex(6)> pid = #PID<0.21.32>

When you do like above, iex shell just wait for more input as #PID<0.21.32> is treated as comment.

Now you enter another data to complete the expression. The entered value is the value of the pid. Lets check that…

iex(6)> pid = #PID<0.21.32>      # here expression is not complete
...(6)> 23    # here we are giving the value 23
23            # expression is complete
iex(7)> pid

9. Replacing the String with global option

The String.replace function will replace the given the pattern with replacing pattern. By default, it replaces all the occurrences of the pattern. Lets check that…

iex(1)> str = ","    

iex(2)> String.replace str,"@","#"

The String.replace str, "@", "#"is same as String.replace str, "@", "#", global: true

But, if you want to replace only the first occurrence of the pattern, you need to pass the option global: false . So, it replaces only the first occurrence of @ . Lets check that…

iex(3)> String.replace str, "@", "#", global: false

Here only first @ is replaced with #.

10.Memory Usage

You can check the memory usage (in bytes) with :erlang.memory

iex(1)> :erlang.memory
[total: 16221568, processes: 4366128, processes_used: 4364992, system: 11855440,
 atom: 264529, atom_used: 250685, binary: 151192, code: 5845369, ets: 331768]

However, you can pass option like :erlang.memory :atom to get the memory usage of atoms.

iex(2)> :erlang.memory :atom

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