What does %User{} = user means?

Questions / Help
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1

Hi, I’m new in ecto and elixir.
Well, I’m learning about phoenix, ecto and found this sample code

def change_registration(%User{} = user, params) do
    User.registration_changeset(user, params)
 end

Does %User{} = user make sense?
I thought the user = %User{} should be right because user is a variable, but
it isn’t in here.

Am I understanding structure and variables wrong?

2

That is called pattern matching. You can read more about it here.

The idea is that the match will pass if the user variable is a %User{} struct and raise an error otherwise. When it’s in a function head like that it’s used to control whether or not the user variable can enter that function. If it’s not a %User{} struct then it won’t be able to go in there. It will look for other versions of change_registration that produce a successful pattern match. If there are none an error will raise.

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3

thanks a lot!

4

A = inside a pattern which is what you are have in your function head just measn that both sides must match. So writing %User{} = user in a pattern means that both the struct %User and the variable user must match. This means that writing user = %User{} in a pattern has the same meaning, both sides must match.

However, and this is a big however, using = in the body of a function has a different meaning. It is a match operator, pattern = expression which first evaluates the expression in RHS and then matches that with the pattern in the LHS. It goes right-to-left.

It was perhaps not so smart to use = in these different ways, but it was inherited fro Erlang :wink:

I hope this hasn’t confused things.

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