Wierd behaviour of parametric type

New to Julia
1

Hello,

I know this is kind of strange usage of parametric types, but I was wondering what’s going on. Sorry for the imprecise title, since I’m not sure how to describe this question.

Without parametric type, if I define a type and implement a wrong way to use it:

struct Foo
    a::String
    b::Int
end

foo = Foo(1, "hello!")

A method error would trigger since one cannot do the incorrect type conversion, that’s what we expect.

However if I use the parametric type in a wierd way:

struct Foo2{String,Int}
    a::String
    b::Int
end

foo2 = Foo2(1, "hello!")

No error would pop up. Instead, an instance Foo2{Int64,String}(1, "hello!") is generated. The specified type on a and b are violated.

What’s going on here? Any help is appreciated!

2

In the second example String and Int are not anymore what they mean in the first case, they are just generic parameters. It is the same as:

struct Foo2{A,B}
  a::A
  b::B
end

(of course nobody would ever use that syntax to specify generic types in a parametric type). In other words, Int and String are not reserved keywords, and here they are just assuming another meaning than the most common one. You can also do String = "abc", and that will work.

4 Likes
3

That totally make sense! Got it, thank you!

1 Like