# Interpolation into Threads.@spawn macro fails for certain kinds of Structs

**URL:** <https://discourse.julialang.org/t/interpolation-into-threads-spawn-macro-fails-for-certain-kinds-of-structs/42964>\
**Category:** Performance\
**Tags:** multithreading, interpolations\
**Created:** [July 13, 2020, 1:17am UTC](https://discourse.julialang.org/t/interpolation-into-threads-spawn-macro-fails-for-certain-kinds-of-structs/42964 "2020-07-13T01:17:06Z")\
**Posts on this page:** 1\
**Showing post:** 4

<div class="post-metadata">

**Author:** ![greg\_plowman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/greg_plowman/32/8100_2.png) [@greg\_plowman](https://discourse.julialang.org/u/greg_plowman)\
**Post date:** [July 13, 2020, 4:39am UTC](https://discourse.julialang.org/t/interpolation-into-threads-spawn-macro-fails-for-certain-kinds-of-structs/42964/4 "2020-07-13T04:39:06Z")

</div>

My understanding is that without interpolation, there is a race condition whereby the object that x refers to might or might not change before the function runs. The interpolation “captures” the binding, guaranteeing that x refers to the object it was bound to at the time of the @spawn macro.

The code below (swapping out func(x) / func($x) demonstrates the difference.

```julia
import Base.Threads.@spawn

function func(A)
    A[1] = 90
    return A
end

const N = 100
is_same_object = Vector{Bool}(undef, N);

for i = 1:N
    x = [1,2,3]
    fx = @spawn func(x)
    #fx = @spawn func($x)
    x = [4,5,6]
    x2 = fetch(fx)
    is_same_object[i] = x2 === x
end

count(is_same_object)

```

If this interpretation is correct, then I think the documentation is misleading:

> Values can be interpolated into @spawn via $, which copies the value directly into the constructed underlying closure. This allows you to insert the value of a variable, isolating the aysnchronous code from changes to the variable’s value in the current task.

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_[View the full topic](https://discourse.julialang.org/t/interpolation-into-threads-spawn-macro-fails-for-certain-kinds-of-structs/42964)._