julia throws: too few type parameters specified in "new{...}"

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I want to make a struct that works something like struct C below, but where C.a and C.b are of the same type:

struct C
    a::Array
    b::Array
    
    function C(a, b)
        length(a) == length(b) || throw(DimensionMismatch())
        new(a, b)
    end
end

I thought this would be trivial to implement by doing the following:

struct D{T<:Int}
    a::Array{T}
    b::Array{T}
    
    function D(a, b)
        length(a) == length(b) || throw(DimensionMismatch())
        new(a, b)
    end
end

But it throws an error:

# syntax: too few type parameters specified in "new{...}" around

I found the beginning of a solution for this on the julia discourse.

struct E{T<:Int}
    a::Array{T}
    b::Array{T}
    
    function E(a::Array{S}, b::Array{S}) where S<:Int
        length(a) == length(b) || throw(DimensionMismatch())
        new{S}(a, b)
    end
end

The error is gone. But for some reason it doesn't work with all Int

v = Array{Int64}([1,2])
w = Array{Int32}([1,2])

E(v, v)
# E{Int64}(Int64[1, 2], Int64[1, 2])

E(v, v)
# MethodError: no method matching Main.workspace321.E(::Array{Int32,1}, ::Array{Int32,1})

Does somebody have an idea what going on here?

julia struct
1 Answers

I found the problem Int is an alias (I think, I couldn't find it in the docs.) for Int64. In any case:

julia> Int32 <: Integer
true

julia> Int32 <: Int
false

julia> Int64 <: Int
true

julia> Int64 == Int 
true

help?> Int
search: Int Int8 Int64 Int32 Int16 Int128 Integer ...

  Int64 <: Signed

  64-bit signed integer type.

So this works:

struct E{T<:Integer}
    a::Array{T}
    b::Array{T}
    function E(a::Array{S}, b::Array{S}) where S<: Integer
        length(a) == length(b) || throw(DimensionMismatch())
        new{S}(a, b)
    end
end
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