# Use of \`Ref\` in broadcasting user-defined functions

**URL:** https://discourse.julialang.org/t/use-of-ref-in-broadcasting-user-defined-functions/64154
**Category:** New to Julia
**Tags:** question
**Created:** [July 6, 2021, 3:17pm UTC](https://discourse.julialang.org/t/use-of-ref-in-broadcasting-user-defined-functions/64154 "2021-07-06T15:17:45Z")
**Posts on this page:** 3
**Page:** 1

<div class="post-metadata">

### Author: ![drarnau](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/drarnau/32/4476_2.png) [@drarnau](https://discourse.julialang.org/u/drarnau)
#### Post date: [July 6, 2021, 3:17pm UTC](https://discourse.julialang.org/t/use-of-ref-in-broadcasting-user-defined-functions/64154/1 "2021-07-06T15:17:45Z")

</div>

Hello,

I would like to apply a function that I’ve created to an array/range. Let me use a simplified version of my code to illustrate my question. It starts defining a structure for parameters, something like:

```julia
using Parameters

@with_kw struct myParams
   α::Float64 = 0.5
   β::Float64 = 0.5
end

p = myParams()

```

Then I create a function, like:

```julia
function myf(x::Float64, p::myParams)::Float64
    @unpack α, β = p

    return β*x^α
end

```

Now I want to apply this function, with a particular instance of `myParams`, to a range or array. I could do it using a `for` loop, but that seems not very julian. Using broadcasting, I thought of writing this:

```julia
@time myf.(range(0.0, stop = 100.0), p)

```

It gives an error:

```julia
MethodError: no method matching length(::myParams)
Closest candidates are:
  length(!Matched::DataStructures.IntSet) at /home/arnau/.julia/packages/DataStructures/ixwFs/src/int_set.jl:191
  length(!Matched::DataStructures.DiBitVector) at /home/arnau/.julia/packages/DataStructures/ixwFs/src/dibit_vector.jl:40
  length(!Matched::ExponentialBackOff) at error.jl:259
  ...
_similar_for(::UnitRange{Int64}, ::Type{Any}, ::myParams, ::Base.HasLength) at array.jl:597
_collect(::UnitRange{Int64}, ::myParams, ::Base.HasEltype, ::Base.HasLength) at array.jl:630
collect(::myParams) at array.jl:624
broadcastable(::myParams) at broadcast.jl:682
broadcasted(::Function, ::StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}, ::myParams) at broadcast.jl:1261
top-level scope at timing.jl:174 [inlined]
top-level scope at Tests.jl:0

```

I’ve tried this, and it works:

```julia
@time myf.(range(0.0, stop = 100.0), Ref(p))

```

However, after reading the documentation for `Ref`, I’m not sure I’m doing the right thing here. Any help?

Thank you.

---

<div class="post-metadata">

### Author: ![Jeff\_Emanuel](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jeff_emanuel/32/15440_2.png) [@Jeff\_Emanuel](https://discourse.julialang.org/u/Jeff_Emanuel)
#### Post date: [July 6, 2021, 3:25pm UTC](https://discourse.julialang.org/t/use-of-ref-in-broadcasting-user-defined-functions/64154/2 "2021-07-06T15:25:15Z")

</div>

You are using Ref in broadcasting correctly. Ref inhibits broadcasting from iterating over the fields of p.

---

<div class="post-metadata">

### Author: ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [July 6, 2021, 4:23pm UTC](https://discourse.julialang.org/t/use-of-ref-in-broadcasting-user-defined-functions/64154/3 "2021-07-06T16:23:46Z")

</div>

If you want your type to always act as a scalar, you can tell the broadcasting machinery to do that:

```julia
Broadcast.broadcastable(mp::MyParams) = Ref(mp)

```

Then `myf.(range, p)` should work.

BTW, it’s a strong convention in Julia to make type names UpperCamelCase. So for clarity, you should probably call your type `MyParams` instead of `myParams`.