# Struct with Parameters.jl and outer function

**URL:** <https://discourse.julialang.org/t/struct-with-parameters-jl-and-outer-function/79886>\
**Category:** General Usage\
**Tags:** package\
**Created:** [April 23, 2022, 10:41am UTC](https://discourse.julialang.org/t/struct-with-parameters-jl-and-outer-function/79886 "2022-04-23T10:41:41Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![Iddingsite](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/iddingsite/32/31840_2.png) [@Iddingsite](https://discourse.julialang.org/u/Iddingsite)\
**Post date:** [April 23, 2022, 10:41am UTC](https://discourse.julialang.org/t/struct-with-parameters-jl-and-outer-function/79886/1 "2022-04-23T10:41:41Z")

</div>

Hi everyone,

I have a simple idea: I want to create a Grid object containing the parameters of my model. I want the user to give values using Unitful to have units and to internally have consistent values without the units. I have managed to do what I want like this for example:

```julia
using Unitful

struct Grid
    nx::Int
    Lx::Float64
    Δx::Float64
    x::StepRangeLen

    function Grid(nx, Lx)
        Δx = Lx / (nx-1)
        x = range(0, length=nx, stop= Lx)
        new(nx, Lx, Δx, x)
    end
end

# convert Lenght unit
function Grid(nx::Int, Lx::Unitful.Length)
    Grid(nx, ustrip(u"m", Lx))
end

grid = Grid(101, 1000u"1000")

```

I am trying to do the same thing using Parameters.jl (to understand it and to see if it can simplify a bit the writing) and I didn’t manage to make it work. Here is my try:

```julia
using Unitful
using Parameters

@with_kw struct Grid
    nx::Int
    Lx::Float64
    Δx::Float64 = Lx / (nx-1)
    x::StepRangeLen = range(0, length=nx, stop= Lx)
end

# convert Lenght unit
function Grid(nx::Int, Lx::Unitful.Length)
    Grid(nx, ustrip(u"m", Lx))
end

grid = Grid(101, 1000u"1000")

```

which yield ERROR: MethodError: no method matching Grid(::Int64, ::Int64)

Is what I am trying to do not possible?

---

<div class="post-metadata">

**Author:** ![goerch](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/goerch/32/29122_2.png) [@goerch](https://discourse.julialang.org/u/goerch)\
**Post date:** [April 25, 2022, 1:30am UTC](https://discourse.julialang.org/t/struct-with-parameters-jl-and-outer-function/79886/2 "2022-04-25T01:30:03Z")

</div>

The following example

```julia
using Unitful
using Parameters

@with_kw struct Grid
    nx::Int
    Lx::Float64
    Δx::Float64 = Lx / (nx-1)
    x::StepRangeLen = range(0, length=nx, stop= Lx)
end

# convert Length unit
function Grid(nx::Int, Lx::Unitful.Length)
    Grid(;nx=nx, Lx=ustrip(u"m", Lx))
end

@show Grid(101, 1000u"m")
@show Grid(101, 1000u"1000")

```

yields

```julia
Grid(101, 1000 * u"m") = Grid
  nx: Int64 101
  Lx: Float64 1000.0
  Δx: Float64 10.0
  x: StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}

ERROR: MethodError: no method matching Grid(::Int64, ::Int64)

```

which seems sensible to me. Your first example seems to break units, because you manage to construct a `Grid` from a non-length unit?

---

<div class="post-metadata">

**Author:** ![Iddingsite](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/iddingsite/32/31840_2.png) [@Iddingsite](https://discourse.julialang.org/u/Iddingsite)\
**Post date:** [April 25, 2022, 10:16am UTC](https://discourse.julialang.org/t/struct-with-parameters-jl-and-outer-function/79886/3 "2022-04-25T10:16:55Z")

</div>

Thx for your reply. I think I did a small mistake when I copy-pasted my code because what I wanted to do first was:

```julia
 Grid(101, 1000u"m")

```

and not

```julia
 Grid(101, 1000u"1000")

```

My mistake in the code was to not write the name of the variables for the input as you should do when using Parameters.jl.

Thx for your help!