# Type issue with captured variables -\> let workaround failed
**URL:** https://discourse.julialang.org/t/type-issue-with-captured-variables-let-workaround-failed/85661
**Category:** General Usage
**Tags:** fftw, performance, closure
**Created:** [August 12, 2022, 12:00pm UTC](https://discourse.julialang.org/t/type-issue-with-captured-variables-let-workaround-failed/85661 "2022-08-12T12:00:00Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@roflmaostc](https://discourse.julialang.org/u/roflmaostc)
#### Post date: [August 12, 2022, 12:00pm UTC](https://discourse.julialang.org/t/type-issue-with-captured-variables-let-workaround-failed/85661/1 "2022-08-12T12:00:00Z")
Hi!
I’m aware of [#15276](https://github.com/JuliaLang/julia/issues/15276) but today I encountered an issue in the proposed [workaround](https://docs.julialang.org/en/v1/manual/performance-tips/#man-performance-captured).
```julia
using FFTW
function f(x)
p = plan_fft(x)
x_ft = p * x
# given workaround in performance tips
g = let p = p
x_ft = x_ft
a -> (p*a) * x_ft
end
return g
end
function f2(x)
p = plan_fft(x)
x_ft = p * x
# difference is a local name which is different to the original x_ft
g = let p = p
x_ft2 = x_ft
a -> (p*a) * x_ft2
end
return g
end
```
The first function struggles with boxing, the second not:
```julia
julia> g = f(x)
#9 (generic function with 1 method)
julia> @code_warntype g(x)
MethodInstance for (::var"#9#10"{FFTW.cFFTWPlan{ComplexF64, -1, false, 2, UnitRange{Int64}}})(::Matrix{Float64})
from (::var"#9#10")(a) in Main at /home/fxw/.julia/dev/FourierTools.jl/lel.jl:11
Arguments
#self#::var"#9#10"{FFTW.cFFTWPlan{ComplexF64, -1, false, 2, UnitRange{Int64}}}
a::Matrix{Float64}
Locals
x_ft::Union{}
Body::Any
1 ─ %1 = Core.getfield(#self#, Symbol("#66#p"))::FFTW.cFFTWPlan{ComplexF64, -1, false, 2, UnitRange{Int64}}
│ %2 = (%1 * a)::Matrix{ComplexF64}
│ %3 = Core.getfield(#self#, :x_ft)::Core.Box
│ %4 = Core.isdefined(%3, :contents)::Bool
└── goto #3 if not %4
2 ─ goto #4
3 ─ Core.NewvarNode(:(x_ft))
└── x_ft
4 ┄ %9 = Core.getfield(%3, :contents)::Any
│ %10 = (%2 * %9)::Any
└── return %10
julia> g2 = f2(x)
#15 (generic function with 1 method)
julia> @code_warntype g2(x)
MethodInstance for (::var"#15#16"{Matrix{ComplexF64}, FFTW.cFFTWPlan{ComplexF64, -1, false, 2, UnitRange{Int64}}})(::Matrix{Float64})
from (::var"#15#16")(a) in Main at /home/fxw/.julia/dev/FourierTools.jl/lel.jl:25
Arguments
#self#::var"#15#16"{Matrix{ComplexF64}, FFTW.cFFTWPlan{ComplexF64, -1, false, 2, UnitRange{Int64}}}
a::Matrix{Float64}
Body::Matrix{ComplexF64}
1 ─ %1 = Core.getfield(#self#, Symbol("#76#p"))::FFTW.cFFTWPlan{ComplexF64, -1, false, 2, UnitRange{Int64}}
│ %2 = (%1 * a)::Matrix{ComplexF64}
│ %3 = Core.getfield(#self#, :x_ft2)::Matrix{ComplexF64}
│ %4 = (%2 * %3)::Matrix{ComplexF64}
└── return %4
```
What is the reason for this behavior? It is definitely related to the FFTW planning or at least to the `FFTW.cFFTWPlan` (which is an own type).
Best,
Felix
---
### Author:  [@digital\_carver](https://discourse.julialang.org/u/digital_carver)
#### Post date: [August 12, 2022, 12:18pm UTC](https://discourse.julialang.org/t/type-issue-with-captured-variables-let-workaround-failed/85661/2 "2022-08-12T12:18:47Z")
Does adding a comma at the end of `let p = p` help? Currently, `x_ft = x_ft` in `f(x)` is the first statement _inside_ the `let` block, not part of its initial comma-separated assignment list.
I don’t understand what `let` does - or scoping in general - well enough to know _why_ that makes a difference. But in my case, it makes both functions have the same `@code_warntype` output with no type issues.
---
### Author:  [@roflmaostc](https://discourse.julialang.org/u/roflmaostc)
#### Post date: [August 12, 2022, 12:20pm UTC](https://discourse.julialang.org/t/type-issue-with-captured-variables-let-workaround-failed/85661/3 "2022-08-12T12:20:52Z")
You’re totally right!
I totally wasn’t aware of that, but it totally makes sense (need urgently fix a few dozens lines in my code 😂)
---
### Author:  [@ikirill](https://discourse.julialang.org/u/ikirill)
#### Post date: [August 13, 2022, 6:59am UTC](https://discourse.julialang.org/t/type-issue-with-captured-variables-let-workaround-failed/85661/4 "2022-08-13T06:59:30Z")
You can look at how Julia parses the blocks:
```julia
julia> using Base.Meta
julia> @dump let a=a
b=b
1
end
Expr
head: Symbol let
args: Array{Any}((2,))
1: Expr
head: Symbol =
args: Array{Any}((2,))
1: Symbol a
2: Symbol a
2: Expr
head: Symbol block
args: Array{Any}((4,))
1: LineNumberNode
line: Int64 2
file: Symbol REPL[2]
2: Expr
head: Symbol =
args: Array{Any}((2,))
1: Symbol b
2: Symbol b
3: LineNumberNode
line: Int64 3
file: Symbol REPL[2]
4: Int64 1
julia> @dump let a=a, b=b
1
end
Expr
head: Symbol let
args: Array{Any}((2,))
1: Expr
head: Symbol block
args: Array{Any}((2,))
1: Expr
head: Symbol =
args: Array{Any}((2,))
1: Symbol a
2: Symbol a
2: Expr
head: Symbol =
args: Array{Any}((2,))
1: Symbol b
2: Symbol b
2: Expr
head: Symbol block
args: Array{Any}((2,))
1: LineNumberNode
line: Int64 2
file: Symbol REPL[3]
2: Int64 1
```
maybe then it’s easier to see what’s wrong. `@dump` helps when the syntax looks ambiguous.