# Why is type inference failing here?

**URL:** <https://discourse.julialang.org/t/why-is-type-inference-failing-here/89803>\
**Category:** Performance\
**Created:** [November 5, 2022, 8:54am UTC](https://discourse.julialang.org/t/why-is-type-inference-failing-here/89803 "2022-11-05T08:54:28Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![robsmith11](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/robsmith11/32/29641_2.png) [@robsmith11](https://discourse.julialang.org/u/robsmith11)\
**Post date:** [November 5, 2022, 8:54am UTC](https://discourse.julialang.org/t/why-is-type-inference-failing-here/89803/1 "2022-11-05T08:54:28Z")

</div>

In the following example, it seems like it should be simple for Julia to infer that the type of `prev_x` is `Float64`. Why is it instead getting assigned a boxed type?

```julia
julia> function tf(xs)
        prev_x = xs[1]
        map(xs) do x
         r = x / prev_x
         prev_x = x
         r
        end
       end
tf (generic function with 1 method)

julia> xs = rand(10^6);

julia> @time tf(xs);
  0.178356 seconds (3.12 M allocations: 61.497 MiB, 67.31% compilation time)

julia> @time tf(xs);
  0.060390 seconds (3.00 M allocations: 53.406 MiB)

julia> @code_warntype tf(xs)
MethodInstance for tf(::Vector{Float64})
  from tf(xs) @ Main REPL[243]:1
Arguments
  #self#::Core.Const(tf)
  xs::Vector{Float64}
Locals
  #228::var"#228#229"
  prev_x::Core.Box
Body::Any
1 ─ (prev_x = Core.Box())
│ %2 = Base.getindex(xs, 1)::Float64
│ Core.setfield!(prev_x, :contents, %2)
│ (#228 = %new(Main.:(var"#228#229"), prev_x))
│ %5 = #228::var"#228#229"
│ %6 = Main.map(%5, xs)::Any
└── return %6

```

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<div class="post-metadata">

**Author:** ![nandoconde](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nandoconde/32/19497_2.png) [@nandoconde](https://discourse.julialang.org/u/nandoconde)\
**Post date:** [November 5, 2022, 9:14am UTC](https://discourse.julialang.org/t/why-is-type-inference-failing-here/89803/2 "2022-11-05T09:14:42Z")

</div>

I think the closure inside `map` is preventing the compiler from accessing type information of the outer variable.

This is a known issue, and there was a discussion some weeks ago about syntax that would allow this kind of information to be handled down.

I am writing from my phone and cannot test this, but try boxing it yourself in a Ref, or to manually annotate type information in closure’s variable like:

```julia
... do x::eltype(xs)
...

```

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<div class="post-metadata">

**Author:** ![robsmith11](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/robsmith11/32/29641_2.png) [@robsmith11](https://discourse.julialang.org/u/robsmith11)\
**Post date:** [November 5, 2022, 12:39pm UTC](https://discourse.julialang.org/t/why-is-type-inference-failing-here/89803/3 "2022-11-05T12:39:36Z")

</div>

Thanks, but looks like that’s currently not allowed:

```julia
julia> function tf(xs)
        prev_x = xs[1]
        map(xs) do x::eltype(xs)
         r = x/prev_x
         prev_x = x
         r
        end
       end
ERROR: syntax: local variable xs cannot be used in closure declaration
Stacktrace:
 [1] top-level scope
   @ REPL[337]:1

```

I also tried `x::Float64`, but that didn’t help with the type inference.

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<div class="post-metadata">

**Author:** ![Elrod](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/elrod/32/22461_2.png) [@Elrod](https://discourse.julialang.org/u/Elrod)\
**Post date:** [November 5, 2022, 2:09pm UTC](https://discourse.julialang.org/t/why-is-type-inference-failing-here/89803/4 "2022-11-05T14:09:42Z")

</div>

Make `prev_x` a `Ref` or use a loop instead.  
I think the loop is cleaner.