# In Julia, how to create a function that saves its own internal state?

**URL:** <https://discourse.julialang.org/t/in-julia-how-to-create-a-function-that-saves-its-own-internal-state/58457>\
**Category:** New to Julia\
**Tags:** question, function\
**Created:** [April 2, 2021, 5:21pm UTC](https://discourse.julialang.org/t/in-julia-how-to-create-a-function-that-saves-its-own-internal-state/58457 "2021-04-02T17:21:33Z")\
**Posts on this page:** 1\
**Showing post:** 10

<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [April 3, 2021, 4:31pm UTC](https://discourse.julialang.org/t/in-julia-how-to-create-a-function-that-saves-its-own-internal-state/58457/10 "2021-04-03T16:31:35Z")

</div>

If I benchmarked everything correctly, the functor with the pure-Int field is by far more efficient. Followed by the functor with the `Union{Nothing,Int}` field:

```julia
julia> include("state.jl")
let: 3.595 μs (0 allocations: 0 bytes)
# functor with `Any` field
f1: 29.342 μs (1000 allocations: 15.62 KiB)
f1_const: 28.184 μs (1000 allocations: 15.62 KiB)
# functor with Union field:
f2: 1.260 μs (0 allocations: 0 bytes)
f2_const: 1.259 μs (0 allocations: 0 bytes)
# functor with pure Int field
f3: 257.421 ns (0 allocations: 0 bytes)
f3_const: 257.424 ns (0 allocations: 0 bytes)

```

> **Code**
>
> ```julia
> 
> let state = Ref{Union{Int, Nothing}}(nothing)
> global f
> function f()
> if state[] !== nothing
> state[] += 1
> else
> state[] = 1
> end
> state[]
> end
> end
> 
> Base.@kwdef mutable struct F1
> state = nothing
> end
> 
> function (o::F1)()
> if o.state == nothing
> o.state = 1
> else
> o.state += 1
> end
> end
> 
> f1 = F1()
> const f1_const = F1()
> 
> Base.@kwdef mutable struct F2
> state::Union{Nothing,Int} = nothing
> end
> 
> function (o::F2)()
> if o.state == nothing
> o.state = 1
> else
> o.state += 1
> end
> end
> 
> f2 = F2()
> const f2_const = F2()
> 
> Base.@kwdef mutable struct F3
> state::Int = 0
> end
> 
> function (o::F3)()
> if o.state == 0
> o.state = 1
> else
> o.state += 1
> end
> end
> 
> f3 = F3()
> const f3_const = F3()
> 
> function update_state!(f,n) 
> for i in 1:n
> f()
> end
> end
> 
> using BenchmarkTools
> n = 1000
> print("let:"); @btime update_state!($f,$n)
> print("f1:"); @btime update_state!($f1,$n)
> print("f1_const:"); @btime update_state!($f1_const,$n)
> print("f2:"); @btime update_state!($f2,$n)
> print("f2_const:"); @btime update_state!($f2_const,$n)
> print("f3:"); @btime update_state!($f3,$n)
> print("f3_const:"); @btime update_state!($f3_const,$n)
> 
> ```

---

_[View the full topic](https://discourse.julialang.org/t/in-julia-how-to-create-a-function-that-saves-its-own-internal-state/58457)._