Elixir: Attributes
Sometimes, in a module, you need to put some values into constants, for this use attributes starting with @ sign.
defmodule MyModule do
@magic_number 8
def do_magic(num) do
num * @magic_number
end
end
MyModule.do_magic(10) # 80
Module attributes are not available from outside the module.
defmodule MyModule do
@magic_number 8
end
MyModule.@magic_number # raises error
Attributes can be declared several times and redefined, but it should be noted that the compiler inlines the last value of the declared attribute, for example:
defmodule MyModule do
@magic_number 8
def cast_magic() do
@magic_number
end
@magic_number 0
def do_magic() do
@magic_number
end
end
MyModule.cast_magic() # 8
MyModule.do_magic() # 0
# after compilation module attributes inlines like this
defmodule MyModule do
@magic_number 8
def cast_magic() do
8
end
@magic_number 0
def do_magic() do
0
end
end
Within modules, there are also special attributes that are used by Elixir to generate documentation, such as the @moduledoc attribute that describes general information about the module or the @doc attribute that documents a declared function:
defmodule MyModule do
@moduledoc "My attributes exercise module."
@magic_number 8
@doc "Do some magic calculations."
def do_magic(num) do
num * @magic_number
end
end
Then, these attributes are used in documentation generation.
You can also make a module attribute accumulate redefined values, with a special attribute declaration, for example:
defmodule MyModule do
Module.register_attribute __MODULE__, :magic_values, accumulate: true
@magic_values 8
@magic_values :some
@magic_values "hello"
def do_magic() do
@magic_values
end
end
MyModule.do_magic # [8, :some, "hello"]
Instructions
Define the following attributes with values inside the module:
- Attribute: number_attr, value: 10
- Attribute: boolean_attr, value: false
- Attribute: hello_world_attr, value: "Hello, World!"
The exercise doesn't pass checking. What to do? 😶
If you've reached a deadlock it's time to ask your question in the «Discussions». How ask a question correctly:
- Be sure to attach the test output, without it it's almost impossible to figure out what went wrong, even if you show your code. It's complicated for developers to execute code in their heads, but having a mistake before their eyes most probably will be helpful.
In my environment the code works, but not here 🤨
Tests are designed so that they test the solution in different ways and against different data. Often the solution works with one kind of input data but doesn't work with others. Check the «Tests» tab to figure this out, you can find hints at the error output.
My code is different from the teacher's one 🤔
It's fine. 🙆 One task in programming can be solved in many different ways. If your code passed all tests, it complies with the task conditions.
In some rare cases, the solution may be adjusted to the tests, but this can be seen immediately.
I've read the lessons but nothing is clear 🙄
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