# Questions about using \`Parameters\`

**URL:** https://discourse.julialang.org/t/questions-about-using-parameters/14378
**Category:** General Usage
**Created:** [August 31, 2018, 6:17pm UTC](https://discourse.julialang.org/t/questions-about-using-parameters/14378 "2018-08-31T18:17:14Z")
**Posts on this page:** 5
**Page:** 1

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### Author: ![briochemc](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/briochemc/32/4209_2.png) [@briochemc](https://discourse.julialang.org/u/briochemc)
#### Post date: [August 31, 2018, 6:17pm UTC](https://discourse.julialang.org/t/questions-about-using-parameters/14378/1 "2018-08-31T18:17:14Z")

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I thought I’d ask some questions about the `Parameters` package here because hopefully the answers will be useful to others. (Maybe some answers actually will make their way to the `Parameters` package documentation?)

Ok, so I work best by example, so here we go, let’s build a `Para` type and a parameter container `p`:

```julia
using Parameters
@with_kw struct Para
    a = 1.0
    b = 3.0 + im
end
p = Para()

```

I put two default values of different types in here. Well first thing I want is to have an `eltype` function for this type, which promotes the type of every parameter (in other words here, I want `eltype(p)` to be the promotion of the types of `a` and `b`). Just below is my current version of overloading `eltype` to do that (please let me know how this could be done better):

```julia
function Base.:eltype(p::Para)
    ot = Int # Starting somewhere 
    pnames = fieldnames(typeof(p))
    for pname in pnames
        ot = promote_type(ot, typeof(getfield(p, pname)))
    end
    return ot
end
eltype(p)

```

Then I want to convert `p` to a vector and back. To convert it to a vector `x`, I do the following (again, this works but I am not sure this is the correct way):

```julia
function para2vec(p::Para)
    pnames = fieldnames(typeof(p))
    np = length(pnames)
    x = zeros(eltype(p), np)
    for ix in 1:np
        x[ix] = getfield(p, pnames[ix])
    end
    return x
end
x = para2vec(p)

```

~~Now the hard part for me is to convert `x` into `p`. Could someone jump in and help with that? I am guessing I could use the `@pack` macro but I haven’t figured out how yet…~~

* * *

**EDIT #1**  
Converting `x` into `p` is in fact just as simple as

```julia
p2 = Para(x...)

```

😄

* * *

**EDIT #2**  
Now I would like to write derivatives of a function `f(u, p::Para)` w.r.t. `p`. (`u` is a vector of element-type `:<Number`.) Right now I do it by hand, i.e., I write

```julia
function Dpf(u, p::Para)
    @unpack a, b = p
    Daf = # derivative of `f` w.r.t `a`
    Dbf = # derivative of `f` w.r.t `b`
    return [Daf Dbf]
end

```

But I am sure there is a better approach, something with more abstraction. I thought of creating a type for those derivatives, i.e. a container like

```julia
struct DerivativeWRTPara
    Daf
    Dbf
end

```

That would contain the same number of fields as the `Para` type (so that there is an “automatic” correspondence between the types). I’m a bit afraid of exploring this avenue: is this stupid?

---

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### Author: ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [September 1, 2018, 5:41am UTC](https://discourse.julialang.org/t/questions-about-using-parameters/14378/2 "2018-09-01T05:41:47Z")

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> [@briochemc](#):
>
> have an `eltype` function for this type, which promotes the type of every parameter (in other words here, I want `eltype(p)` to be the promotion of the types of `a` and `b` )

This is not `Parameters`-specific advice (I love `Parameters.@unpack`, but don’t really use anything else), but I would have a type parameter, and have the constructor promote it.

```julia
struct Para{T}
    a::T
    b::T
end

```

This will allow the compiler to optimize your code. Then

```julia
Base.eltype(::Para{T}) where T = T

```

For conversion, I would define an iterator, so I could also do

```julia
a, b = Para(1, 2)

```

and `collect`. See the example for [`Base.Pair`](https://github.com/JuliaLang/julia/blob/8acad0ceca8e8c58a01727daccb1c69b158cdaf7/base/pair.jl#L49).

Converting _from_ a vector should be as easy as `Para(v...)`.

> [@briochemc](#):
>
> That would contain the same number of fields as the `Para` type (so that there is an “automatic” correspondence between the types). I’m a bit afraid of exploring this avenue: is this stupid?

Hard to say without context. I would just use automatic differentiation, eg

> **[GitHub - JuliaDiff/ForwardDiff.jl: Forward Mode Automatic Differentiation for...](https://github.com/JuliaDiff/ForwardDiff.jl)**
>
> Forward Mode Automatic Differentiation for Julia. Contribute to JuliaDiff/ForwardDiff.jl development by creating an account on GitHub.

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### Author: ![mauro3](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mauro3/32/292_2.png) [@mauro3](https://discourse.julialang.org/u/mauro3)
#### Post date: [September 1, 2018, 3:20pm UTC](https://discourse.julialang.org/t/questions-about-using-parameters/14378/3 "2018-09-01T15:20:31Z")

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I don’t think `eltype` is the right function to overload here, unless you view your type as some sort of iterable collection. Also, Tamas’ advice is good: parameterize the type, see [Performance Tips · The Julia Language](https://docs.julialang.org/en/v1/manual/performance-tips/#Avoid-containers-with-abstract-type-parameters-1).

---

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### Author: ![briochemc](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/briochemc/32/4209_2.png) [@briochemc](https://discourse.julialang.org/u/briochemc)
#### Post date: [September 1, 2018, 11:05pm UTC](https://discourse.julialang.org/t/questions-about-using-parameters/14378/4 "2018-09-01T23:05:00Z")

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Thanks, I’ll try to implement all that and repot back

---

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### Author: ![briochemc](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/briochemc/32/4209_2.png) [@briochemc](https://discourse.julialang.org/u/briochemc)
#### Post date: [October 3, 2018, 7:16am UTC](https://discourse.julialang.org/t/questions-about-using-parameters/14378/5 "2018-10-03T07:16:43Z")

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So I have made the changes you suggested and it works nicely, so thanks 🙂