# Trying to understand the return type of an array comprehension if the function being called returns a Union

**URL:** https://discourse.julialang.org/t/trying-to-understand-the-return-type-of-an-array-comprehension-if-the-function-being-called-returns-a-union/63424
**Category:** General Usage
**Tags:** inference, array, code\_warntype, type-stability
**Created:** [June 23, 2021, 8:48am UTC](https://discourse.julialang.org/t/trying-to-understand-the-return-type-of-an-array-comprehension-if-the-function-being-called-returns-a-union/63424 "2021-06-23T08:48:39Z")
**Posts on this page:** 2
**Page:** 1

<div class="post-metadata">

### Author: ![jishnub](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jishnub/32/33620_2.png) [@jishnub](https://discourse.julialang.org/u/jishnub)
#### Post date: [June 23, 2021, 8:48am UTC](https://discourse.julialang.org/t/trying-to-understand-the-return-type-of-an-array-comprehension-if-the-function-being-called-returns-a-union/63424/1 "2021-06-23T08:48:39Z")

</div>

On nightly:

```julia
julia> f(x) = x > 3 ? x : nothing
f (generic function with 1 method)

julia> g(imax) = [f(i) for i in 1:imax]
g (generic function with 1 method)

julia> @code_warntype g(5)
MethodInstance for g(::Int64)
  from g(imax) in Main at REPL[7]:1
Arguments
  #self#::Core.Const(g)
  imax::Int64
Locals
  #5::var"#5#6"
Body::Union{Vector{Nothing}, Vector{Union{Nothing, Int64}}, Vector{Int64}}
1 ─ (#5 = %new(Main.:(var"#5#6")))
│ %2 = #5::Core.Const(var"#5#6"())
│ %3 = (1:imax)::Core.PartialStruct(UnitRange{Int64}, Any[Core.Const(1), Int64])
│ %4 = Base.Generator(%2, %3)::Core.PartialStruct(Base.Generator{UnitRange{Int64}, var"#5#6"}, Any[Core.Const(var"#5#6"()), Core.PartialStruct(UnitRange{Int64}, Any[Core.Const(1), Int64])])
│ %5 = Base.collect(%4)::Union{Vector{Nothing}, Vector{Union{Nothing, Int64}}, Vector{Int64}}
└── return %5

```

The return type here may be chosen to be `Vector{Union{Nothing, Int}}` instead of the union of types. I wonder if this is intentional, expecting union splitting in downstream usage? Or is this something that may be improved upon?

---

<div class="post-metadata">

### Author: ![gustaphe](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gustaphe/32/18174_2.png) [@gustaphe](https://discourse.julialang.org/u/gustaphe)
#### Post date: [June 23, 2021, 9:02am UTC](https://discourse.julialang.org/t/trying-to-understand-the-return-type-of-an-array-comprehension-if-the-function-being-called-returns-a-union/63424/2 "2021-06-23T09:02:43Z")

</div>

Guessing here: there is a chance the function returns a `Vector{Int}` (I.e. `imax.>3`). It would be a waste of potential to treat that as a `Vector{Union}`.