# Constructor functionality within Struct

**URL:** <https://discourse.julialang.org/t/constructor-functionality-within-struct/99040>\
**Category:** General Usage\
**Tags:** function, struct\
**Created:** [May 18, 2023, 5:46am UTC](https://discourse.julialang.org/t/constructor-functionality-within-struct/99040 "2023-05-18T05:46:01Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Lincoln\_Hannah](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lincoln_hannah/32/19198_2.png) [@Lincoln\_Hannah](https://discourse.julialang.org/u/Lincoln_Hannah)\
**Post date:** [May 18, 2023, 5:46am UTC](https://discourse.julialang.org/t/constructor-functionality-within-struct/99040/1 "2023-05-18T05:46:01Z")

</div>

Is this bad coding?

```julia
@with_kw struct X @deftype Float64

    a; b; c; d; e; f; g

    xv::Vector{Float64}

    yv = map(xv) do x
       # 10 lines of calculation using struct variables a - g.
    end

end

```

The `do` block could be replaced with an external function, it wouldn’t be used anywhere else and I wanted to avoid passing the half dozen variables to it.

Will the functionality of the do block be copied for every instance of the struct ?  
Or just the result - `yv` (this would be preferable).

---

<div class="post-metadata">

**Author:** ![Sevi](https://avatars.discourse-cdn.com/v4/letter/s/c67d28/32.png) [@Sevi](https://discourse.julialang.org/u/Sevi)\
**Post date:** [May 18, 2023, 7:27am UTC](https://discourse.julialang.org/t/constructor-functionality-within-struct/99040/2 "2023-05-18T07:27:28Z")

</div>

I personally always need a second more to understand `do` blocks than “normal” functions, but I think you have good reasons to do it like that – depending on how complex your actual struct is, this is the easiest to read for me, but I would also be curious about other ways to approach this.

But regarding your second question: With `@macroexpand` you can check the exact code that will be generated and see what will actually be stored in the struct:

> **expanded macro**
>
> (I removed everything after the struct definition and added some empty lines to make it more readable; also the calculation of `ys` is just a dummy of course)
> 
> ```julia
> @macroexpand @with_kw struct X @deftype Float64
> a; b; c; d; e; f; g
> xv::Vector{Float64}
> 
> yv::Vector{Float64} = map(xv) do x
> x * a + b * c + d * e * f * g
> end
> end
> 
> begin
> $(Expr(:meta, :doc))
> struct X
> a::Float64
> b::Float64
> c::Float64
> d::Float64
> e::Float64
> f::Float64
> g::Float64
> xv::Vector{Float64}
> "Default: map(xv) do x\n #= REPL[24]:8 =#\n x * a + b * c + d * e * f * g\nend"
> yv::Vector{Float64}
> 
> X(; a = #= remaining fields, edited out =# , yv = map(xv) do x
> #= REPL[24]:8 =#
> x * a + b * c + d * e * f * g
> end) = begin
> 
> X(a, b, c, d, e, f, g, xv, yv)
> end
> 
> X(a, b, c, d, e, f, g, xv, yv) = begin
> new(a, b, c, d, e, f, g, xv, yv)
> end
> end
> end
> 
> ```

So the function to generate `ys` is “stored” just inside the constructor like `X(; ..., ys = anonymous_fn(...))`, but the struct just contains the plain values that come out of the computation. Note that I had to annotate `ys::Vector{Float64}` because of the `@deftype Float64`.

---

<div class="post-metadata">

**Author:** ![Lincoln\_Hannah](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lincoln_hannah/32/19198_2.png) [@Lincoln\_Hannah](https://discourse.julialang.org/u/Lincoln_Hannah)\
**Post date:** [May 18, 2023, 8:36am UTC](https://discourse.julialang.org/t/constructor-functionality-within-struct/99040/3 "2023-05-18T08:36:53Z")

</div>

Thank you. Nice explanation. and desired outcome 🙂