Update element on map depending on other update - dictionary

Question

I want to do update a map doing the following:
def updateInfo() do
person = %{ person | name : "new name", age : person.age +1, observation : changeObs(person.age)}
end
def changeObs(age), when age >= 18, do: "Adult"
def changeObs(age), do: "kid"
If I updateInfo() and the age is 17, I would expect the person to change the observation to "Adult". But it is not working. I thought the updates of the map were done sequentially, but apparently not, so I cannot rely on the fact that the age now is 18. I can do this if I split the update like so:
person = %{ person | name : "new name", age : person.age +1}
person = %{ person | observation : changeObs(person.age)}
Is there a way to keep it all the update in one line, relying on previous updates of the attributes on the map?

Answer 1

Well, as far as I can tell, you can just cache the new value in a variable:
new_age = person.age + 1
person = %{ person | name: "new name", age: new_age, observation: changeObs(new_age)}
Or you could piple the changes like this:
person
|> Map.put(:name, "new_name")
|> Map.put(:age, new_age)
|> Map.put(:observation, changeObs(new_age))

Answer 2

First of all, the syntax of updating maps uses colons, not equal signs. Your code raises SyntaxError exception.
There are plenty of ways to accomplish the task as oneliner:
person = with age <- person.age + 1,
do: %{person | age: age,
observation: (if age >= 18, do: "Adult", else: "Kid")}
Or:
person = (age = person.age + 1;
%{person | age: age,
observation: (if age >= 18, do: "Adult", else: "Kid")})
Or a pipe chain:
{_, person} = person
|> Map.put(:age, person.age + 1)
|> Map.get_and_update(:age, fn age ->
{age, age + 1}
end)
The idiomatic would be the last one. My fave would be the first one.