Part- 7

10|> Pretty Printing quoted Expression

Macro.to_string |> IO.puts

In general, when you pass the quote expression to Macro.to_string , it returns the actual code in a string format and we all knew this.

The weird thing is, it gives the output in a single line along with characters inside the string.

Check it down

    iex(2)> q = quote do
    ...(2)> 1+2
    ...(2)> name = "blackode"
    ...(2)> end

    {:__block__, [],
     [
       {:+, [context: Elixir, import: Kernel], [1, 2]},
       {:=, [], [{:name, [], Elixir}, "blackode"]}
     ]}

    iex(3)> Macro.to_string q
    "(\n  1 + 2\n  name = \"blackode\"\n)"

To print the new lines, you pipe the string output from Macro.to_string to IO.puts like in the following code snippet. It gives the clean output by printing in new line.

    iex(4)> Macro.to_string(q) |> IO.puts
    (
      1 + 2
      name = "blackode"
    )
    :ok

Check this in live

9|> Finding the loaded files in iex and dynamic loading files on-fly

1. Finding the loaded files

Code.loaded_files

In elixir, we are having a definition loaded_files in the module Code used to find the loaded files.

Let’s see this.

Run the command iex inside your terminal and call the function Code.loaded_files .

It gives an empty list [] as output because we haven’t loaded any files so far.

    $ iex
    iex> Code.loaded_files
    []

File Loading

Let’s create a file hello.ex with a single function definition hello which just prints a string “Welcome to the world” for the demo purpose.

    # hello.ex

    defmodule Hello do
      def hello do
        IO.puts "Welcome to the world"
      end
    end

Save the file after editing.

Now, make sure you are in the same directory of where the file exists and run the command

    $ iex hello.ex

It opens the Elixir interactive shell by loading the hello.ex file as well.

Now, you can see the loaded file by calling Code.loaded_files . It outputs the path to the file.

    iex> Code.loaded_files
    ["/home/john/hello.ex"]

That is one way of loading files. You can also load them on-fly in iex session

Dynamic loading on-fly

Code.load_file/1

Unlike loading files with iex hello.ex , you can load them on-fly in iexsession with the help of Code.load_file "path/to/file" .

Let us assume that we have a file hello.ex in our current directory and open iex session from the same directory.

    $ iex

    iex> Code.load_file "hello.ex"
    [
      {Hello,
       <<70, 79, 82, 49, 0, 0, 4, 72, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 0, 140,
         0, 0, 0, 15, 12, 69, 108, 105, 120, 105, 114, 46, 72, 101, 108, 108, 111,
         8, 95, 95, 105, 110, 102, 111, 95, 95, 9, ...>>}
    ]

Let's check this in action

8|> Providing deprecation reason with

@deprecated

You might be noticed the warnings of using deprecated functions in a library along with some useful hint text. Today, we build this from scratch.

Suppose, you have written a library and want to update the name of one function in your next build release and if the user tried to access the old function then you have to warn him of using deprecated function instead of updated one.

To see this in action, you need to create new mix project.

Let’s do that.

    mix new hello

Next change the directory to the project created.

    cd hello

Now, edit the file lib/hello.ex in the project with the following code

    #lib/hello.ex

    defmodule Hello do
      def hello do 
        Printee.print()
      end
    end

    defmodule Printee do

      @deprecated "print/0 is deprecated use show/0" 
      def print do
        IO.puts "hello blackode"
      end

      def show do
        IO.puts "hello blackode"
      end

    end

File Scrennshot Vim Editor

This file comprises of two modules Hello and Printee . The Printee module comprises of two functions print/0 and show/0 . Here purposely, print/0 is considered as a deprecated function used inside the Hello module.

The mix compiler automatically looks for calls to deprecated modules and emit warnings during compilation.

So, when you compile the project mix compile, it gives a warning saying

“print/0 is deprecated use show/0”

like in the following screenshot.

Deprecated Function Warning

Check the live coding

7|> Module Creation with create function

Module.create/3

As we all know defmodule is used to create a module. In similar, Module.create/3 is also used to create a module.

The only difference in defining a module with Module.create/3 is the definition inside the module is quoted expression.

However, we can use the help of quote to create a quoted expression.

We have to pass three parameters to Moduel.create .

The first one is name of the module. The second one is module definition in quoted expression. The third one is location. The location is given in keyword list like [file: String.t, line: Integer ] or else you can just take the help of Macro.Env.location(__ENV__) which returns the same.

image

location of loc

    iex> module_definition = quote do: def hello, do: IO.puts "hello"

The above line of code gives the context of the module definition and collected in module_definition . We can use this module_defintion to pass as second parameter to Module.create/3 function.

module definition context

Let’s put all together and see the magic

    iex(8)> Module.create Hello, module_definition, [file: "iex", line: 8 ]

image

Creating another module Foo with same module function

    iex(10)> Module.create Foo, module_definition, Macro.Env.location(__ENV__)

image

Check out the live execution

6|> Finding the list of bound values in iex

binding

    iex> binding
    []
    iex> name = "blackode"
    "blackode"
    iex> blog = "medium"
    "medium"
    iex> binding
    [blog: "medium", name: "blackode"]

image

This is used in debugging purpose inside the function.

It is also very common to use IO.inspect/2 with binding(), which returns all variable names and their values:

def fun(a, b) do
  IO.inspect binding()
  ...
end

When fun/3 is invoked with :laughing, "time" it prints:

[a: :laughing, b: "time"]

You can also give context to the binding

If the given context is nil *(by default it is), *the binding for the current context is returned.

    iex> var!(x, :foo) = 1
    1
    iex> binding(:foo)
    [x: 1]

Check out the live execution

5|> Fun with Char_Lists

We can convert any integer to charlist using Integer.char_list . It can be used in two ways either passing the base the base value or not passing.

Let’s try with base and with out base.

without base

    iex> Integer.to_charlist(882681651)
    '882681651'

with base

    iex> Integer.to_charlist(882681651, 36)
    'ELIXIR'

Try your own names as well and find your value with base

    iex> List.to_integer 'BLACKODE', 36
    908344015970

    iex> Integer.to_charlist(908344015970, 36)
    'BLACKODE'

4|> Extracting Process Information

With the help of Process.info , we can extract the process information like linked processes etc…

It is used in two different ways.

Note: *We get the information if and only if process is alive*

Here, we try to create a process and linking it to the current process using spawn_link and will try to extract only linked processes. With no surprise, it should give the self output pid which is the current process in our case.

    iex> pid = spawn_link fn -> receive do :name -> IO.puts "Hello Medium" end end
    #PID<0.210.0>
    iex> Process.info pid, :links                                                 
    {:links, [#PID<0.85.0>]}
    iex> self
    #PID<0.85.0>

image

Extracting whole information

    iex> Process.info pid

image

3|> Forcing Keys in a Map parameter in functions

Consider that you need to pass a map to a function and you have to ensure certain keys in the map then allow it to use the function body.

You can achieve this using structs with @enforce_keys attribute. But, you can still use the pattern matching to the keys of a map.

    defmodule Hello do
      def hello(%{name: _name, blog: _blog} = map) do
        IO.inspect map
      end
    end

Here, we don’t care how many keys present inside the map but we need atleast two keys name, blog to be present inside the map with any values.

Execution screenshot

image

If you observer the screenshot, we tried to access the function by sending a map with single key parameter where it is not allowed then with two keys map and the keys are exactly pattern matched where it is allowed to use the function then evenutally tried with more keys still it worked.

Thanks to pattern matching.

2|> Universal Code base formatter

Code formatting is easy now with the help of mix task mix format . It will take care of all you care about cleaning and refactoring as well. It can assure a clean code base or simply universal code base which maintains some useful standards. This really saves a lot of time.

    mix format filename

To know more about the codebase formatting, check out my recent article on

[Code Formatter The Big Feature in Elixir

1.6](https://medium.com/blackode/code-formatter-the-big-feature-in-elixir-v1-6-0-f6572061a4ba)

which explains all about using mix format task and its configuration in detail with screen shots of vivid examples.

1|> Alias multiple modules in one line

    alias Mod.{One, Two, Three}

is same as following

    alias Mod.One
    alias Mod.Two
    alias Mod.Three

In similar fashion, you can alias your current module like

    defmodule User.Authentication do
      defstruct [:key, :token]

      alias __MODULE__, as: Auth

      .....
    end

In general, with out using alias __MODULE__ , we have to type full module name like %User.Authentication{key: "key", token: ".."} to define the struct but now simply %Auth{key: "key", token: ".."} will be the better approach.

Happy Coding!

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