Manipulating a struct in an existing variable - struct

Question

Complete Elixir beginner. I am sure I am missing some fundamental issues in the following. Can someone please explain, exactly why the second code not work as the first one?
defmodule Bulb do
defstruct [:state]
def turn_on(bulb = %Bulb{}) do
%{bulb | state: :on}
end
def turn_off(bulb = %Bulb{}) do
%{bulb | state: :off}
end
end
bulb = %Bulb{state: :on} # --> %Bulb{state: :on}
bulb |> Bulb.turn_off() # --> %Bulb{state: :off}
bulb |> Bulb.turn_on() # --> %Bulb{state: :on}
versus
defmodule Bulb do
defstruct [:state]
def switch_bulb(bulb = %Bulb{}) do
case bulb do
%{state: :off} -> %{bulb | state: :on}
%{state: :on} -> %{bulb | state: :off}
end
end
end
bulb = %Bulb{state: :on} # --> %Bulb{state: :on}
bulb |> Bulb.switch_bulb() # --> %Bulb{state: :off}
bulb |> Bulb.switch_bulb() # --> %Bulb{state: :off} why?
(Yes, the second one also works if I do the following.)
defmodule Bulb do
defstruct [:state]
def switch_bulb(bulb = %Bulb{}) do
case bulb do
%{state: :off} -> %{bulb | state: :on}
%{state: :on} -> %{bulb | state: :off}
end
end
end
bulb = %Bulb{state: :on} # --> %Bulb{state: :on}
bulb = bulb |> Bulb.switch_bulb() # --> %Bulb{state: :off}
bulb = bulb |> Bulb.switch_bulb() # --> %Bulb{state: :on}

Answer 1

In elixir, everything is immutable. bulb |> Bulb.turn_on() from the first snippet does not do what you think it does. It does literally nothing, actually, “switching” the state from on (initial value) to on.
To understand, what happens, do print bulb every time.
bulb = %Bulb{state: :on} # %Bulb{state: :on}
bulb |> IO.inspect() |> Bulb.turn_off() |> IO.inspect() # on → off
bulb |> IO.inspect() |> Bulb.turn_on() |> IO.inspect() # on → on
bulb from inside switch_bulb/1 function has the same name, but a very different scope, it’s not the same variable. bulb stays what it was assigned to.
bulb = %Bulb{state: :on} # --> %Bulb{state: :on}
IO.inspect(bulb) # --> %Bulb{state: :on}
bulb |> Bulb.switch_bulb() # --> %Bulb{state: :off}
bulb # --> %Bulb{state: :on}
bulb |> Bulb.switch_bulb() # --> %Bulb{state: :off} why?
Even more: the example below won’t change the value of outermost bulb too
bulb = :on
if true, do: bulb = :off # different scope, outermost `bulb` is untouched
IO.inspect(bulb) #⇒ :on
That said unless you reassign the value of any expression, it simply gets lost. Nothing is mutated in place.
BTW, = is NOT an assignment, it’s a matching operator. In this case, it’s rebinding the variable.