# Get generic constructor of parametric type
**URL:** https://discourse.julialang.org/t/get-generic-constructor-of-parametric-type/57189
**Category:** New to Julia
**Tags:** question, parametric-types, constructors
**Created:** [March 15, 2021, 11:41am UTC](https://discourse.julialang.org/t/get-generic-constructor-of-parametric-type/57189 "2021-03-15T11:41:57Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@MatFi](https://discourse.julialang.org/u/MatFi)
#### Post date: [March 15, 2021, 11:41am UTC](https://discourse.julialang.org/t/get-generic-constructor-of-parametric-type/57189/1 "2021-03-15T11:41:57Z")
Is there a smart /easy way to get the generic constructor of a parametric type from its instance? I actually thought that this is a standard thing, but couldn’t find anything.
```julia
abstract type AbstractFooBar end
struct Foo{T} <: AbstractFooBar
p::T
end
struct Bar{T} <: AbstractFooBar
p::T
end
#= more subtypes
struct X{T} <: AbstractFooBar end
struct Y{T} <: AbstractFooBar end
.
=#
function get_generic_constructor(fb::AbstractFooBar)
fb isa Bar && return Bar
fb isa Foo && return Foo
#= Would like to avoid doing this for all subtybes of AbstractFooBar
fb isa X && return X ...
=#
end
```
What I want to archive is the following behavior:
```julia
a = Bar(3)
#Bar{Int64}(3)
b = get_generic_constructor(Bar(3))
#Bar
b("three")
#Bar{String}("three")
```
I need this in my code to make it compatible to ForwardDiff.jl. So please my favorite community, enlighten me
---
### Author:  [@Benny](https://discourse.julialang.org/u/Benny)
#### Post date: [March 15, 2021, 11:59am UTC](https://discourse.julialang.org/t/get-generic-constructor-of-parametric-type/57189/2 "2021-03-15T11:59:05Z")
Another way of saying what you want to do is to get the UnionAll type if it exists. You can get it with `typeof(a).name.wrapper`.
That line can also take types without parameters, but in those cases, `typeof(a) === typeof(a).name.wrapper`
---
### Author:  [@MatFi](https://discourse.julialang.org/u/MatFi)
#### Post date: [March 15, 2021, 12:13pm UTC](https://discourse.julialang.org/t/get-generic-constructor-of-parametric-type/57189/3 "2021-03-15T12:13:52Z")
Thanks,
`typeof(a).name.wrapper` was what I was looking for.
Since I never saw this anywhere before and I encountered this problem already multiple times, I wonder if I do something fundamentally wrong. However thanks a lot…
---
### Author:  [@Philippe\_Maincon1](https://discourse.julialang.org/u/Philippe_Maincon1)
#### Post date: [August 8, 2021, 11:49am UTC](https://discourse.julialang.org/t/get-generic-constructor-of-parametric-type/57189/4 "2021-08-08T11:49:08Z")
Great, I had the same question and found the answer here. Whatever you do fundamentally, wrong, so do I !!!
---
### Author:  [@Henrique\_Becker](https://discourse.julialang.org/u/Henrique_Becker)
#### Post date: [August 8, 2021, 5:03pm UTC](https://discourse.julialang.org/t/get-generic-constructor-of-parametric-type/57189/5 "2021-08-08T17:03:53Z")
> [@Benny](#):
>
> `typeof(a).name.wrapper`
I think this solution depends on implementation details and, therefore, may break in any change of Julia versions. As far as I know, the founders/‘language designers’ have not decided on a way to do what you want to do, or even if having such feature is desirable.
---
### Author:  [@MatFi](https://discourse.julialang.org/u/MatFi)
#### Post date: [August 8, 2021, 5:47pm UTC](https://discourse.julialang.org/t/get-generic-constructor-of-parametric-type/57189/6 "2021-08-08T17:47:34Z")
Do you know if there is an open issue about that somewhere?. Because I can think of several workarounds for this, but they all suffer from a lack of generically and I would like to know more about the pros and cons regarding such a feature.
---
### Author:  [@j\_verzani](https://discourse.julialang.org/u/j_verzani)
#### Post date: [August 8, 2021, 6:04pm UTC](https://discourse.julialang.org/t/get-generic-constructor-of-parametric-type/57189/7 "2021-08-08T18:04:40Z")
Is this helpful [https://github.com/JuliaObjects/ConstructionBase.jl/blob/master/src/constructorof.md](https://github.com/JuliaObjects/ConstructionBase.jl/blob/master/src/constructorof.md)?
---
### Author:  [@MatFi](https://discourse.julialang.org/u/MatFi)
#### Post date: [August 8, 2021, 6:26pm UTC](https://discourse.julialang.org/t/get-generic-constructor-of-parametric-type/57189/8 "2021-08-08T18:26:31Z")
Absolutely!! The whole magic is actually done only by these lines:
```julia
@generated function constructorof(::Type{T}) where T
getfield(parentmodule(T), nameof(T))
end
constructorof(typeof(a)) # Bar
```
---
### Author:  [@mcabbott](https://discourse.julialang.org/u/mcabbott)
#### Post date: [August 8, 2021, 7:58pm UTC](https://discourse.julialang.org/t/get-generic-constructor-of-parametric-type/57189/9 "2021-08-08T19:58:09Z")
> [@MatFi](#):
>
> Do you know if there is an open issue
I think you want [issue 35543](https://github.com/JuliaLang/julia/issues/35543).