Confused with composite type constructors

New to Julia
1

Hello, I am new to Julia, I would like to understand language more deeply, so I have several question:
(I am using 1.12.x version)

First of all, where can I find some BNF like definition?

Another question, I tried to create this composite type (struct}

struct OMG2{X<: Number, Y<: String}
    a::Int128
    b::Int128

    function OMG2(x::T) where T<:Float64
        println("Float64")
        
      
    end

    function OMG2(x::T, y::U) where {T<: Int64,U <: Int64}
        println("Int64, Int64")

    end

    function OMG2(x::T, y::U) where {T<: Float32,U <: Float32}
        println("Float32, Float32")
   

    end

end

I don’t understand why if the constructors are within the struct the var types X, Y are not propagated to constructors:
Also seems that the constructors don’t have access to the var a and b

struct OMG2{X<: Number, Y<: String}
    a::Int128
    b::Int128

    function OMG2(x::Y)
        # Wont work <----
        println("THIS WONT WORK")
    end

    function OMG2(x::T) where T<:Float64
        println("Any")
      
      
    end

    function OMG2(x::T, y::U) where {T<: Int64,U <: Int64}
        println("Int64, Int64")


    end

    function OMG2(x::T, y::U) where {T<: Float32,U <: Float32}
        println("Float32, Float32")
   

    end

end

I would like to have some kind of restriction, in inner constructors, but seems that this restriction is applied only if I have something like that (constructors are defined out of the composite types, and I don’t understand why):

struct OMG3{X<: Number, Y<: String}
    a::Int128
    b::Int128


    function OMG3(x::T) where T<:Float64
        println("Float64")
        
    end
end



# Created constructor out of the struct, and I don't understand that the conditions {X<:Number, Y<: String, is applied just for this kind of constructor
julia> OMG3{Int64, String}() = println("Int64, String")

# Seems that it is is basically some kind of instance
julia> OMG3{Int64, String}.

flags       hash
instance    layout
name        parameters
super       types
julia> OMG3{Int64, String}.instance


# Constructor in the struct, seems that it is "empty"
julia> OMG3{Float64}.

body
var



# So, I have 2 constructors, but if I do:
julia> methods(OMG3)
# 1 method for type constructor:
 [1] OMG3(x::T) where T<:Float64
     @ ~/devel/julia/base64proj/base64test.jl:9

# I will see just the struct one, and I don't understand why.
# From one side, I have restrictions based on my <:, and that constructor is not included in the methods output (it is created outside of the struct), and from other side I have struct constructor, but I can change the type restriction and seems, that it doesn't have access to the struct attributes like a, b

I am confused can somebody help me?

2

Please don’t take this the wrong way, but there seems to be a lot of confusion. Have you read the manual section about constructors (in your case particularly the Parametric section)?
I think that it can answer a lot of the easier doubts.

Having said that what are you trying to achieve? Are a and b initialized independently from the inputs?