# Index inside a comprehension is boxed

**URL:** <https://discourse.julialang.org/t/index-inside-a-comprehension-is-boxed/9315>\
**Category:** General Usage\
**Created:** [February 25, 2018, 2:01pm UTC](https://discourse.julialang.org/t/index-inside-a-comprehension-is-boxed/9315 "2018-02-25T14:01:32Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![BenLauwens](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/benlauwens/32/2051_2.png) [@BenLauwens](https://discourse.julialang.org/u/BenLauwens)\
**Post date:** [February 25, 2018, 2:01pm UTC](https://discourse.julialang.org/t/index-inside-a-comprehension-is-boxed/9315/1 "2018-02-25T14:01:32Z")

</div>

Hi

I got a bug report for `ResumableFunctions` and after some analysis the problem appeared to be the boxing of an integer used as an index in a comprehension.

Minimal example showing the boxing of the variable `i`

```julia
function test0()
  a = [[1, 2],[3, 4]]
  for i in 1:2 
    u = [v[i] for v in a]
  end
end

```

```julia
code_warntype(test0,())
Variables:
  #self# <optimized out>
  #69::##69#70
  i::Core.Box
  u <optimized out>
  #temp#::Int64
  a::Array{Array{Int64,1},1}

Body:
  begin
      a::Array{Array{Int64,1},1} = $(Expr(:invoke, MethodInstance for vect(::Array{Int64,1}, ::Vararg{Array{Int64,1},N} where N), :(Base.vect), :($(Expr(:invoke, MethodInstance for vect(::Int64, ::Vararg{Int64,N} where N), :(Base.vect), 1, 2))), :($(Expr(:invoke, MethodInstance for vect(::Int64, ::Vararg{Int64,N} where N), :(Base.vect), 3, 4))))) # line 3:
      SSAValue(5) = (Base.select_value)((Base.sle_int)(1, 2)::Bool, 2, (Base.sub_int)(1, 1)::Int64)::Int64
      #temp#::Int64 = 1
      5:
      unless (Base.not_int)((#temp#::Int64 === (Base.add_int)(SSAValue(5), 1)::Int64)::Bool)::Bool goto 19
      i::Core.Box = $(Expr(:new, :(Core.Box)))
      SSAValue(6) = #temp#::Int64
      SSAValue(7) = (Base.add_int)(#temp#::Int64, 1)::Int64
      SSAValue(2) = SSAValue(6)
      (Core.setfield!)(i::Core.Box, :contents, SSAValue(2))::Int64
      #temp#::Int64 = SSAValue(7) # line 3:
      #69::##69#70 = $(Expr(:new, :(Main.##69#70), :(i)))
      SSAValue(4) = $(Expr(:new, Base.Generator{Array{Array{Int64,1},1},##69#70}, :(#69), :(a)))
      $(Expr(:invoke, MethodInstance for collect(::Base.Generator{Array{Array{Int64,1},1},##69#70}), :(Base.collect), SSAValue(4)))
      17:
      goto 5
      19:
      return
  end::Void

```

Is this normal? I don’t see why the boxing is needed.  
If no `for` loop is used, there is no boxing and if `i` is not used as an index in a comprehension, there is also no boxing…

---

<div class="post-metadata">

**Author:** ![antoine-levitt](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/antoine-levitt/32/4008_2.png) [@antoine-levitt](https://discourse.julialang.org/u/antoine-levitt)\
**Post date:** [February 25, 2018, 2:17pm UTC](https://discourse.julialang.org/t/index-inside-a-comprehension-is-boxed/9315/2 "2018-02-25T14:17:47Z")

</div>

This is a known (very annoying) issue: [performance of captured variables in closures · Issue #15276 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/15276)

---

<div class="post-metadata">

**Author:** ![tkoolen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkoolen/32/1603_2.png) [@tkoolen](https://discourse.julialang.org/u/tkoolen)\
**Post date:** [February 25, 2018, 2:41pm UTC](https://discourse.julialang.org/t/index-inside-a-comprehension-is-boxed/9315/3 "2018-02-25T14:41:37Z")

</div>

The `let` block workaround works here too:

```julia
function test0()
    a = [[1, 2],[3, 4]]
    for i in 1:2
        u = let i = i
            [v[i] for v in a]
        end
    end
end

```

---

<div class="post-metadata">

**Author:** ![BenLauwens](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/benlauwens/32/2051_2.png) [@BenLauwens](https://discourse.julialang.org/u/BenLauwens)\
**Post date:** [February 25, 2018, 2:52pm UTC](https://discourse.julialang.org/t/index-inside-a-comprehension-is-boxed/9315/4 "2018-02-25T14:52:27Z")

</div>

Thanks!  
The package [ResumableFunctions](https://github.com/BenLauwens/ResumableFunctions.jl.git) does not support `let` blocks yet. So I have to implement the use of `let` blocks or find another workaround;)

---

<div class="post-metadata">

**Author:** ![tkoolen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkoolen/32/1603_2.png) [@tkoolen](https://discourse.julialang.org/u/tkoolen)\
**Post date:** [February 25, 2018, 8:56pm UTC](https://discourse.julialang.org/t/index-inside-a-comprehension-is-boxed/9315/5 "2018-02-25T20:56:30Z")

</div>

You could try using [https://github.com/c42f/FastClosures.jl](https://github.com/c42f/FastClosures.jl) as well.

---

<div class="post-metadata">

**Author:** ![BenLauwens](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/benlauwens/32/2051_2.png) [@BenLauwens](https://discourse.julialang.org/u/BenLauwens)\
**Post date:** [February 26, 2018, 10:34pm UTC](https://discourse.julialang.org/t/index-inside-a-comprehension-is-boxed/9315/6 "2018-02-26T22:34:51Z")

</div>

I implemented `let` blocks in [ResumableFunctions](https://github.com/BenLauwens/ResumableFunctions.jl.git).

```julia
@resumable function test_let()
  for u in [[(1,2),(3,4)], [(5,6),(7,8)]]
    for i in 1:2
      let i=i
        val = [a[i] for a in u]
      end
      @yield val
    end
  end
end

```

```julia
collect(test_let())

[[1,3],[2,4],[5,7],[6,8]]

```

The variables in a `let` block are not stored between function calls. This is a nice way of improving performance;)  
Thanks for the hints!

Ben