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In chapter 7 we learned about maps:

iex> map = %{a: 1, b: 2}
%{a: 1, b: 2}
iex> map[:a]
1
iex> %{map | a: 3}
%{a: 3, b: 2}

Structs are extensions built on top of maps that provide compile-time checks and default values.

Defining structs

To define a struct, the defstruct construct is used:

iex> defmodule User do
...>   defstruct name: "John", age: 27
...> end

The keyword list used with defstruct defines what fields the struct will have along with their default values.

Structs take the name of the module they're defined in. In the example above, we defined a struct named User.

We can now create User structs by using a syntax similar to the one used to create maps:

iex> %User{}
%User{age: 27, name: "John"}
iex> %User{name: "Meg"}
%User{age: 27, name: "Meg"}

Structs provide compile-time guarantees that only the fields (and all of them) defined through defstruct will be allowed to exist in a struct:

iex> %User{oops: :field}
** (CompileError) iex:3: unknown key :oops for struct User

Accessing and updating structs

When we discussed maps, we showed how we can access and update the fields of a map. The same techniques (and the same syntax) apply to structs as well:

iex> john = %User{}
%User{age: 27, name: "John"}
iex> john.name
"John"
iex> meg = %{john | name: "Meg"}
%User{age: 27, name: "Meg"}
iex> %{meg | oops: :field}
** (KeyError) key :oops not found in: %User{age: 27, name: "Meg"}

When using the update syntax (|), the VM is aware that no new keys will be added to the struct, allowing the maps underneath to share their structure in memory. In the example above, both john and meg share the same key structure in memory.

Structs can also be used in pattern matching, both for matching on the value of specific keys as well as for ensuring that the matching value is a struct of the same type as the matched value.

iex> %User{name: name} = john
%User{age: 27, name: "John"}
iex> name
"John"
iex> %User{} = %{}
** (MatchError) no match of right hand side value: %{}

Structs are bare maps underneath

In the example above, pattern matching works because underneath structs are just bare maps with a fixed set of fields. As maps, structs store a "special" field named __struct__ that holds the name of the struct:

iex> is_map(john)
true
iex> john.__struct__
User

Notice that we referred to structs as bare maps because none of the protocols implemented for maps are available for structs. For example, you can neither enumerate nor access a struct:

iex> john = %User{}
%User{age: 27, name: "John"}
iex> john[:name]
** (UndefinedFunctionError) undefined function: User.fetch/2
iex> Enum.each john, fn({field, value}) -> IO.puts(value) end
** (Protocol.UndefinedError) protocol Enumerable not implemented for %User{age: 27, name: "John"}

However, since structs are just maps, they work with the functions from the Map module:

iex> kurt = Map.put(%User{}, :name, "Kurt")
%User{age: 27, name: "Kurt"}
iex> Map.merge(kurt, %User{name: "Takashi"})
%User{age: 27, name: "Takashi"}
iex> Map.keys(john)
[:__struct__, :age, :name]

Structs alongside protocols provide one of the most important features for Elixir developers: data polymorphism. That's what we will explore in the next chapter.