part- 5

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Killer Elixir-Tips
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part- 5
1. Fetching the default Mix Compilers list
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iex> Mix.compilers
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Returns the default compilers used by Mix. The output will look something similar to [:yecc, :leex, :erlang, :elixir, :xref, :app] .
It can be used in your mix.exs to prepend or append new compilers to Mix:
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#mix.exs
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def project do
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 [compilers: Mix.compilers ++ [:gettext]
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end
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2. Picking out the elements in List
We all know that a proper list is a combination of head and tail like [head | tail] . We can use the same principle for picking out the elements in the list like the following way…
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iex> [first | [second | [third | [ fourth | _rest ]]]] = [1, 2, 3, 4, 5, 6, 7]
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[1, 2, 3, 4, 5, 6, 7]
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iex> first
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1
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iex> {second, third, fourth}
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{2, 3, 4}
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iex(5)>
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We can also use simplified syntax for the same job:
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iex> [first, second, third, fourth | _rest] = [1, 2, 3, 4, 5, 6, 7]
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[1, 2, 3, 4, 5, 6, 7]
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iex> first
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1
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iex> {second, third, fourth}
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{2, 3, 4}
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3. get_in /Access.all()
We all know that the get_in function is used to extract the key which is deeper inside the map by providing the list with keys like a following way…
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iex> user = %{"name" => {"first_name" => "blackode", "last_name" => "john" }}
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%{"name" => %{"first_name" => "blackode", "last_name" => "john"}}
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iex > get_in user, ["name", "first_name"]
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"blackode"
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But, if there is a list of maps [maps] where you have to extract first_name of the each map, generally we go for enum . We can also achieve this by using the get_in and Access.all()
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iex> users=[%{"user" => %{"first_name" => "john", "age" => 23}},
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            %{"user" => %{"first_name" => "hari", "age" => 22}},
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            %{"user" => %{"first_name" => "mahesh", "age" => 21}}]
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# that is a list of maps 
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iex> get_in users, [Access.all(), "user", "age"]
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[23, 22, 21]
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iex> get_in users, [Access.all(), "user", "first_name"]
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["john", "hari", "mahesh"]
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Note: If the key is not present in map, then it returns nil Warning: When you use the get_in along with Access.all() , as the first value in the list of keys like above, the users datatype should be list. If you pass the map, it returns the error.
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iex(17)> list = [%{name: "john"}, %{name: "mary"}, %{age: 34}]
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[%{name: "john"}, %{name: "mary"}, %{age: 34}]
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iex(18)> get_in(list, [Access.all(), :name])
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["john", "mary", nil]
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In the above lines of code returns the nil for key which is not in the map.
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iex(19)> get_in(%{name: "blackode"}, [Access.all(), :name])
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** (RuntimeError) Access.all/0 expected a list, got: %{name: "blackode"}
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    (elixir) lib/access.ex:567: Access.all/3
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In the above lines of code returns the error for passing map .
However, you can change the position of the Access.all() in the list. But the before key should return the list. Check the following lines of code.
Deep Dive
We can also use the Access.all() with functions like update_in, get_and_update_in, etc..
For instance, given a user with a list of books, here is how to deeply traverse the map and convert all book names to uppercase:
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iex> user = %{name: "john", books: [%{name: "my soul", type: "tragedy"}, %{name: "my heart", type: "romantic"}, %{name: "my enemy", type: "horror"}]}
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iex> update_in user, [:books, Access.all(), :name], &String.upcase/1
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%{books: [%{name: "MY SOUL", type: "tragedy"}, %{name: "MY HEART", type: "romantic"}, %{name: "MY ENEMY", type: "horror"}], name: "john"}
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iex> get_in user, [:books, Access.all(), :name]
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["my soul", "my heart", "my enemy"]
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Here, user is not a list unlike in the previous examples where we passed the users as a list. But, we changed the position of Access.all() and inside the list of keys [:books, Access.all(), :name], the value of the key :books should return the list, other wise it raises an error.
4. Data Comprehension along with filters
We achieve the data comprehension through for x <- [1, 2, 3], do: x + 1 . But we can also add the comprehension along with filter.
General Usage
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iex> for x <- [1, 2, 3, 4], do: x + 1
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[2, 3, 4, 5]
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# that is how we use in general lets think out of the box
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With filters
Here I am using two lists of numbers and cross product over the lists and filtering out the product which is a odd number.
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iex> for x <- [1, 2, 3, 4], y <- [5, 6, 7, 8], rem(x * y, 2) == 0, do: {x, y, x * y}
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[{1, 5, 5}, {1, 7, 7}, {3, 5, 15}, {3, 7, 21}]
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#here rem(x * y, 2) is acting as a filter.
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5. Comprehension with binary strings.
Comprehension with binary is little different. You supposed to wrap inside <<>>
Lets check that…
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iex> b_string = <<"blackode">>
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"blackode"
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iex> for << x <- b_string >>, do: x + 1
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'cmbdlpef'
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#here it is printing out the letter after every letter in the "blackode"
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Did you observe that x <- b_string is just changed something like << x <- b_string >> to make the sense.
6. Advanced Comprehension IO.stream
Here we are taking the elixir comprehension to the next level.
We read the input from the keyboard and convert that to upcase and after that it should wait for another entry.
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for x <- IO.stream(:stdio, :line), into: IO.stream(:stdio, :line), do: String.upcase(x)
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Basically IO.stream(:stdio, :line) will the read a line input from the keyboard.
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iex> for x <- IO.stream(:stdio, :line), into: IO.stream(:stdio, :line), do: String.upcase(x)
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hello
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HELLO
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hi
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HI
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who are you?
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WHO ARE YOU?
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blackode
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BLACKODE
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^c ^c # to break
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7. Single Line Multiple module aliasing
We can also alias multiple modules in one line:
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alias Hello.{One,Two,Three}
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#The above line is same as the following 
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alias Hello.One
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alias Hello.Two
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alias Hello.Three
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8. Importing Underscore Functions
By default the functions with _ are not imported. However, you can do that by importing them with :only explicitly.
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import File.Stream, only: [__build__: 3]
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9. Sub string in Elixir
There is no direct sub_str like function in elixir. However you can achieve that by String.slice/2
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iex> String.slice("blackode", 1..-1)
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"lackode"
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iex> String.slice("blackode", 0..-4)
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"black"
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10. String Concatenation
We can do the string concatenation in two ways.
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iex> str1 = "hello"
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iex> str2 = "blackode"
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I am taking above lines of code for example…
String Interpolation
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iex> mystring = "#{str1}#{str2}"
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helloblackode
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Using <> operator
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iex> mystring = str1 <> str2
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helloblackode
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This is the best style and recommended one.
If you are having the list of strings ["hello", "blackode"] then use Enum.join
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iex> mystrings = ["hello", "blackode"]
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["hello", "blackode"]
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iex> Enum.join(mystrings)
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"helloblackode"
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Contents
1. Fetching the default Mix Compilers list
2. Picking out the elements in List
3. get_in /Access.all()
4. Data Comprehension along with filters
5. Comprehension with binary strings.
6. Advanced Comprehension IO.stream
7. Single Line Multiple module aliasing
8. Importing Underscore Functions
9. Sub string in Elixir
10. String Concatenation