We ran into an issue today with structs that I found a bit surprising.

The essence of the issue is:

• A struct is a Map under the hood
• you can manipulate a struct with Map functions, deleting keys that should be there and adding keys that shouldn’t
• as long as the __struct__ key is still there, the Map is treated as a struct including in Elixir pattern matching

Here’s some code which demonstrates this:

defmodule Sandbox do    
  @enforce_keys [:mandatory]
  defstruct [:mandatory, :optional]

  def struct_foo do
    s1 = %Sandbox{mandatory: "must have this"}
    do_something("s1", s1)

    s2 = Map.delete(s1, :mandatory)
    do_something("s2", s2)

    s3 = Map.put(s1, :bogus, "blah")
    do_something("s3", s3)


  end

  def do_something(s, %Sandbox{} = thing) do
    IO.puts "#{s} #{inspect thing} is a %Sandbox"
  end

  def do_something(s, thing) do
    IO.puts "#{s} #{inspect thing} is just a thing"
  end

end

iex(24)> Sandbox.struct_foo()
s1 %Sandbox{mandatory: "must have this", optional: nil} is a %Sandbox
s2 %{__struct__: Sandbox, optional: nil} is a %Sandbox
s3 %{__struct__: Sandbox, bogus: "blah", mandatory: "must have this", optional: nil} is a %Sandbox

One interesting thing is that IO.inspect seems to know the difference between the original struct and one that’s been hacked into a Map, but Elixir pattern matching doesn’t.

So you almost certainly should not be manipulating a struct in this way, and you certainly shouldn’t be deleting keys from structs. The question is: should (could) Elixir do anything to stop you doing this? Should Map functions check for __struct__ and behave differently if they are asked to do something nefarious to a struct? Should a Map function which manipulates a struct at the very least also remove the __struct__ key so it returns a Map that is not treated as a struct any more?