15 Structs

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In chapter 7 we learned about maps:

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

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

15.1 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

15.2 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
iex> meg = %{john | name: "Meg"}
%User{age: 27, name: "meg"}
iex> %{meg | oops: :field}
** (ArgumentError) argument error

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
iex> %User{} = %{}
** (MatchError) no match of right hand side value: %{}

15.3 Structs are just 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)
iex> john.__struct__

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’t enumerate nor access a struct:

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

A struct also is not a dictionary and therefore can’t be used with the functions from the Dict module:

iex> Dict.get(%User{}, :name)
** (UndefinedFunctionError) undefined function: User.fetch/2

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]

We will cover how structs interact with protocols in the next chapter.