Macro-defined callable structs `ArgumentError: invalid type for argument number 1 in method definition`

General Usage
,
2 
julia> @macroexpand @definestruct(MyStruct, 6)
quote
    #= REPL[1]:3 =#
    struct MyStruct{var"#13#T"}
        #= REPL[1]:4 =#
        v::Main.Vector{var"#13#T"}
    end
    #= REPL[1]:6 =#
    (var"#11#MyStruct"(var"#14#v"::Main.Vector{var"#15#T"}) where var"#15#T") = begin
            #= REPL[1]:6 =#
            var"#11#MyStruct"{var"#15#T"}(var"#14#v")
        end
    #= REPL[1]:7 =#
    var"#12#nvars"(::var"#11#MyStruct") = begin
            #= REPL[1]:7 =#
            6
        end
end

as you can see, the (var"#11#MyStruct"(var"#14#v"::Main.Vector{var"#15#T"}) where var"#15#T") does not define something on the MyStruct.

I think you want to escape this:

julia> macro definestruct(name, ndims)
           return esc(quote
               struct $name{T}
                   v::Vector{T}
               end
               $name(v::Vector{T}) where T = $name{T}(v)
               nvars(::$name) = $ndims
           end)
       end
@definestruct (macro with 1 method)

julia> @macroexpand @definestruct(MyStruct, 6)
quote
    #= REPL[4]:3 =#
    struct MyStruct{T}
        #= REPL[4]:4 =#
        v::Vector{T}
    end
    #= REPL[4]:6 =#
    (MyStruct(v::Vector{T}) where T) = begin
            #= REPL[4]:6 =#
            MyStruct{T}(v)
        end
    #= REPL[4]:7 =#
    nvars(::MyStruct) = begin
            #= REPL[4]:7 =#
            6
        end
end
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