# Convenient way to create a copy of an immutable struct with a changed field
**URL:** https://discourse.julialang.org/t/convenient-way-to-create-a-copy-of-an-immutable-struct-with-a-changed-field/28899
**Category:** General Usage
**Created:** [September 18, 2019, 4:02pm UTC](https://discourse.julialang.org/t/convenient-way-to-create-a-copy-of-an-immutable-struct-with-a-changed-field/28899 "2019-09-18T16:02:51Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@tamasgal](https://discourse.julialang.org/u/tamasgal)
#### Post date: [September 18, 2019, 4:02pm UTC](https://discourse.julialang.org/t/convenient-way-to-create-a-copy-of-an-immutable-struct-with-a-changed-field/28899/1 "2019-09-18T16:02:51Z")
It might sound a bit weird but I am facing the following issue quite often:
Given a largish struct:
```julia
struct Foo
a
b
c
d
e
f
g
h
i
j
end
```
And a bunch of them in a `Vector`
```julia
foos = [Foo(rand(10)...) for _ ∈ 1:10]
```
I’d like to “alter” e.g. one field (e.g. subtracting 23 from each `.h`), by creating an exact copy with the new values of `.h`.
Something like
```julia
altered_foos = copy(foos, but = :h=>x->x-23)
```
Is there a somewhat convenient method/package or whatever to do this other than writing out the a new constructor? I’d like to avoid code repetition and also tried it via meta programming but I have the feeling this has been solved before.
Edit: no, I don’t want to make it mutable 😉
---
### Author:  [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)
#### Post date: [September 18, 2019, 4:03pm UTC](https://discourse.julialang.org/t/convenient-way-to-create-a-copy-of-an-immutable-struct-with-a-changed-field/28899/2 "2019-09-18T16:03:35Z")
[https://github.com/jw3126/Setfield.jl](https://github.com/jw3126/Setfield.jl) maybe.
---
### Author:  [@tamasgal](https://discourse.julialang.org/u/tamasgal)
#### Post date: [September 18, 2019, 4:03pm UTC](https://discourse.julialang.org/t/convenient-way-to-create-a-copy-of-an-immutable-struct-with-a-changed-field/28899/3 "2019-09-18T16:03:56Z")
That was fast 😂
---
### Author:  [@tamasgal](https://discourse.julialang.org/u/tamasgal)
#### Post date: [September 18, 2019, 4:04pm UTC](https://discourse.julialang.org/t/convenient-way-to-create-a-copy-of-an-immutable-struct-with-a-changed-field/28899/4 "2019-09-18T16:04:40Z")
Awesome, it’s like lenses in haskell. Thanks!