# Non-mutating array functions
**URL:** https://discourse.julialang.org/t/non-mutating-array-functions/115948
**Category:** Internals & Design
**Created:** [June 20, 2024, 7:11pm UTC](https://discourse.julialang.org/t/non-mutating-array-functions/115948 "2024-06-20T19:11:08Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@jar1](https://discourse.julialang.org/u/jar1)
#### Post date: [June 20, 2024, 7:11pm UTC](https://discourse.julialang.org/t/non-mutating-array-functions/115948/1 "2024-06-20T19:11:08Z")
I sometimes want non-mutating versions of `push!` and `append!` which copy their mutable inputs. [Another thread](https://discourse.julialang.org/t/non-mutating-deleteat-splice-function/11139) asks about non-mutating versions of `splice!` and `deleteat!`.
Do we already have these somewhere? Can we add them to Base?
---
### Author:  [@Benny](https://discourse.julialang.org/u/Benny)
#### Post date: [June 20, 2024, 8:30pm UTC](https://discourse.julialang.org/t/non-mutating-array-functions/115948/2 "2024-06-20T20:30:01Z")
concatenation?
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### Author:  [@greatpet](https://discourse.julialang.org/u/greatpet)
#### Post date: [June 20, 2024, 8:30pm UTC](https://discourse.julialang.org/t/non-mutating-array-functions/115948/3 "2024-06-20T20:30:58Z")
Yes, the non-mutating version of `append!` is simply `vcat`.
Here’s a non-mutating version of `deleteat!`:
```julia
julia> using Accessors: delete, @optic
julia> delete([3,4], @optic _[1]) == [4]
true
```
---
### Author:  [@nsajko](https://discourse.julialang.org/u/nsajko)
#### Post date: [June 20, 2024, 8:36pm UTC](https://discourse.julialang.org/t/non-mutating-array-functions/115948/4 "2024-06-20T20:36:02Z")
>
>
> I want a non-mutating version of \`append!\` that puts two sequences together.
>
> …
> \`\`\`jl
> julia\> (append(x::T, y::T) where T\<: AbstractVector) = append!(copy(x), y);
>
> julia\> (append(x::T, y::T) where T\<:AbstractString)= join(\[x, y\]);
>
> julia\> append(\[:a, :b\], \[:c, :d\])
> 4-element Vector{Symbol}:
> :a
> :b
> :c
> :d
>
> julia\> append("ab", "cd")
> "abcd"
> \`\`\`
>
>
> When the argument types differ we could have some promotion logic.
>
>
> Here are the properties implied by the "sequence" requirement. They should hold unless one of the functions on the left hand side throws an error.
>
>
> \- \`append(eachindex(a), eachindex(b)) == eachindex(append(a,b))\`.
> \- \`((x in a) || (x in b)) iff (x in append(a,b))\`
> \- \`i isa Integer implies a\[i\] == append(a,b)\[i\]\`
> \- \`i isa Integer implies b\[i\] == append(a,b)\[length(a)+i\]\`
>
> I think that means for \`Matrix\` arguments it's \`hcat\` and for \`Set\` it's \`union\`, for \`Tuple\` it does the obvious thing. For \`NamedTuple\` it can be defined just like \`Tuple\` unless \`any(map(==, keys(a), keys(b)))\`, in which case it can error or last wins (I prefer the error, probably). I'm not sure what to say about \`Dict\` and \`Dictionary\`. This could be weakened to also support generators; idk if that's worth doing.
---
### Author:  [@jar1](https://discourse.julialang.org/u/jar1)
#### Post date: [June 20, 2024, 8:36pm UTC](https://discourse.julialang.org/t/non-mutating-array-functions/115948/5 "2024-06-20T20:36:09Z")
haha I forgot that’s my issue
---
### Author:  [@aplavin](https://discourse.julialang.org/u/aplavin)
#### Post date: [June 20, 2024, 9:08pm UTC](https://discourse.julialang.org/t/non-mutating-array-functions/115948/6 "2024-06-20T21:08:49Z")
> [@greatpet](#):
>
> ```julia
> julia> delete([3,4], @optic _[1]) == [4]
>
> ```
It’s even nicer in practice:
```julia
x = [3, 4]
y = @delete x[1]
```
For `push!`, there’s of course `@insert` (:
```julia
y = @insert last(x) = 5
```