# Mutating a mutable field of an immutable and exclamation mark
**URL:** https://discourse.julialang.org/t/mutating-a-mutable-field-of-an-immutable-and-exclamation-mark/21279
**Category:** New to Julia
**Tags:** question
**Created:** [February 27, 2019, 8:45pm UTC](https://discourse.julialang.org/t/mutating-a-mutable-field-of-an-immutable-and-exclamation-mark/21279 "2019-02-27T20:45:31Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Pavel\_Kalouguine](https://discourse.julialang.org/u/Pavel_Kalouguine)
#### Post date: [February 27, 2019, 8:45pm UTC](https://discourse.julialang.org/t/mutating-a-mutable-field-of-an-immutable-and-exclamation-mark/21279/1 "2019-02-27T20:45:31Z")
Should one decorate the name of the function that modifies a mutable field of an immutable structure with an exclamation mark? For instance, should one call the function in the example below “optimize!” or “optimize”?
```julia
struct SomeData
x::Int
end
struct Augmentation
y::Vector{Int}
end
Augmentation()=Augmentation([0])
struct AugmentedData
s::SomeData
a::Augmentation
end
AugmentedData(s::SomeData)=AugmentedData(s, Augmentation())
function optimize!(ad::AugmentedData)
ad.a.y[1]=42
nothing
end
julia> ad=AugmentedData(s)
AugmentedData(SomeData(1), Augmentation([0]))
julia> optimize!(ad)
julia> ad
AugmentedData(SomeData(1), Augmentation([42]))
```
---
### Author:  [@Elrod](https://discourse.julialang.org/u/Elrod)
#### Post date: [February 27, 2019, 8:46pm UTC](https://discourse.julialang.org/t/mutating-a-mutable-field-of-an-immutable-and-exclamation-mark/21279/2 "2019-02-27T20:46:36Z")
`optimize!`, because you are mutating the contents.
---
### Author:  [@Pavel\_Kalouguine](https://discourse.julialang.org/u/Pavel_Kalouguine)
#### Post date: [February 27, 2019, 8:48pm UTC](https://discourse.julialang.org/t/mutating-a-mutable-field-of-an-immutable-and-exclamation-mark/21279/3 "2019-02-27T20:48:50Z")
This is what I think, too. But it looks a bit weird for a function taking a single immutable argument.