# Why is this snippet of code type instable

**URL:** https://discourse.julialang.org/t/why-is-this-snippet-of-code-type-instable/11442
**Category:** General Usage
**Tags:** type
**Created:** [June 5, 2018, 12:55pm UTC](https://discourse.julialang.org/t/why-is-this-snippet-of-code-type-instable/11442 "2018-06-05T12:55:22Z")
**Posts on this page:** 4
**Page:** 1

<div class="post-metadata">

### Author: ![stakaz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stakaz/32/4740_2.png) [@stakaz](https://discourse.julialang.org/u/stakaz)
#### Post date: [June 5, 2018, 12:55pm UTC](https://discourse.julialang.org/t/why-is-this-snippet-of-code-type-instable/11442/1 "2018-06-05T12:55:22Z")

</div>

Hello, I have the following very “simple” code

```julia
f(Λ, inds) where {T,N} = Λ[inds...] *= -1 
x = [1,2,3,4]
@code_warntype f(x, [1:2])

```

But this seams to be type unstable:

```julia
Variables:
  #self# <optimized out>
  Λ::Array{Int64,1}
  inds::Array{UnitRange{Int64},1}

Body:
  begin
      (Core._apply)(test.setindex!, (Core.tuple)(Λ::Array{Int64,1}, -1)::Tuple{Array{Int64,1},Int64}, inds::Array{UnitRange{Int64},1})::Any
      return -1
  end::Int64
nothing

```

Why is it so? And what would be the right way to multiply (apply a function) on **some** values of an Array without allocation and with type stability… I cannot figure it out.

---

<div class="post-metadata">

### 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: [June 5, 2018, 1:09pm UTC](https://discourse.julialang.org/t/why-is-this-snippet-of-code-type-instable/11442/2 "2018-06-05T13:09:01Z")

</div>

Must be some other method that is left over from an earlier experiment, since

```julia
julia> f(Λ, inds) where {T,N} = Λ[inds...] *= -1
WARNING: static parameter T does not occur in signature for f at REPL[5]:1.
The method will not be callable.
WARNING: static parameter N does not occur in signature for f at REPL[5]:1.
The method will not be callable.
f (generic function with 1 method)

julia> x = [1,2,3,4]
4-element Array{Int64,1}:
 1
 2
 3
 4

julia> f(x, [1:2])
ERROR: MethodError: no method matching f(::Array{Int64,1}, ::Array{UnitRange{Int64},1})
Closest candidates are:
  f(::Any, ::Any) where {T, N} at REPL[5]:1

```

However, I think that the crux of the issue (if you remove the `where` clause) is that you are splicing (`...`) an _ex ante_ unknown number of indices to the arguments of `getindex` (`[]`), so the signature cannot be determined in advance.

---

<div class="post-metadata">

### Author: ![stakaz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stakaz/32/4740_2.png) [@stakaz](https://discourse.julialang.org/u/stakaz)
#### Post date: [June 5, 2018, 1:11pm UTC](https://discourse.julialang.org/t/why-is-this-snippet-of-code-type-instable/11442/3 "2018-06-05T13:11:32Z")

</div>

Ah, yeah, the where argument was indeed from the test before…

I understand the issue but is there another way of doing it?

---

<div class="post-metadata">

### Author: ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)
#### Post date: [June 5, 2018, 1:26pm UTC](https://discourse.julialang.org/t/why-is-this-snippet-of-code-type-instable/11442/4 "2018-06-05T13:26:25Z")

</div>

How about a loop?

```julia
julia> function f(x, inds)
         for i in inds
           x[i] *= -1
         end
       end
f (generic function with 1 method)

julia> using BenchmarkTools

julia> @btime f($[1,2,3,4], 1:2)
  3.713 ns (0 allocations: 0 bytes)

```

or a `view()`, which should be pretty fast in v0.7.