Variables in Julia change their type while being constructed - constructor

Question

I am brand new in Julia programming, and am facing a baffling situation, which I simply can't understand. So, this is what I want to do: I have two structs: Shoe and Person, where Person contains a Shoe
This is how the code looks:
struct Shoe
size :: Int64
brand :: String
end
struct Person
age :: Int64
shoe :: Shoe
function Person(a::Int64, s::Shoe)
age = a
shoe = s
end
function Person()
age = 33
shoe = Shoe(17,"Nike")
end
end
ian = Person(17, Shoe(32,"Puma"))
tom = Person()
println(typeof(ian))
println(typeof(tom))
To my utter surprise, ian and tom, which I try to define as persons, turn out to be shoes. Any idea what am I doing wrong?

Answer 1

function Person, being a constructor for the struct Person, should return an instance of struct Person, but because its last line is shoe = Shoe(17,"Nike") (and because assignment is an expression that evaluates to whatever shoe becomes), it ends up returning that Shoe.
Constructors should call the built-in new constructor:
struct Person
age :: Int64
shoe :: Shoe
# This function is redundant since
# it does exactly the same thing as the `new` constructor
function Person(a::Int64, s::Shoe)
age = a
shoe = s
new(age, shoe)
end
function Person()
age = 33
shoe = Shoe(17,"Nike")
new(age, shoe)
end
end
AFAIK, you don't need to use constructors unless they're doing input validation. You do need a constructor to ensure that it's impossible to construct a Person with negative age, for example, because Int64 can be negative. (You could ensure that age >= 0 by using UInt64, too. But then you'd have to write 17 in hexadecimal (0x11) all the time, which is kind of annoying)
I'd write it like this:
struct Person
age :: Int64
shoe :: Shoe
function Person(age::Int64, shoe::Shoe)
#assert age >= 0
new(age, shoe)
end
end
# Put other constructors outside
Person() = Person(33, Shoe(17, "Nike"))
Example:
julia> Person(-1, Shoe(0, "Hello"))
ERROR: AssertionError: age >= 0
Stacktrace:
[1] Person(age::Int64, shoe::Shoe)
# Main ./REPL[3]:6
[2] top-level scope
# REPL[5]:1
julia> Person()
Person(33, Shoe(17, "Nike"))