How do I define a structure inside a function?

New to Julia
1

Hello,
I want to define a structure inside a function:

function PrintDate(d::Date)
    struct Date 
        day::Int64
        month::String
        year::Int64
    end
    println(d.day,d.month,d.year)
end

chmas = Date(25,"Dec",2025)

PrintDate(chmas)

But I got the following error:

ERROR: LoadError: syntax: "struct" expression not at top level

How to solve it?

Thank you.

2

I have never seen a structure defined inside a function. Is there a reason why you don’t want to define it outside?

1 Like
3

Why not define the structure outside of the function?

julia> struct Date 
           day::Int64
           month::String
           year::Int64
       end

julia> function PrintDate(d::Date)
       println(d.day,d.month,d.year)
       end
PrintDate (generic function
 with 1 method)

julia> chmas = Date(25,"Dec",2025)
Date(25, "Dec", 2025)

julia> PrintDate(chmas)
25Dec2025

If you want to create structures on the fly, consider using named tuples

julia> chmas = (day=25,month="Dec",year=2025)
(day = 25, month = "Dec", year = 2025)


julia> function PrintDate(d)
       println(d.day,d.month,d.year)
       end
PrintDate (generic function with 2 methods)

julia> PrintDate(chmas)
25Dec2025
5 Likes
4

Hello,
Thank you for your answers.
So, is this impossible?

5

Yes. It’s says so. “not at top level”. You can however run top level stuff inside a function with eval or @eval:

function f()
    @eval struct Foo
        a::Int
    end
end

and even create it with you own name

function g(name)
    @eval struct $(Symbol(name))
        a::Int
    end
end
g("MyStruct")
3 Likes
6

The question sounds like an XY problem.

I suspect that you are trying to solve a specific problem, for which defining a structure in a function seemed like a good solution. If you share the original problem here, you are likely going to get much better suggestions.

10 Likes
7

Hello,
I’m learning Julia and here I’ll ask questions that come to my mind.

2 Likes
8

Of course any questions are welcome. What @barucden meant to say is that, from the perspective of people who have been using Julia for a while, if you feel like you need to define a struct inside a function, you may need to rethink your approach.

11 Likes
9

In particular, most use cases for a struct defined in a function are covered either by named tuples, as mentioned, or parametric types.

3 Likes