# Silent type promotion when creating range using \`a🅱c\`

**URL:** <https://discourse.julialang.org/t/silent-type-promotion-when-creating-range-using-ac/127671>\
**Category:** General Usage\
**Created:** [April 3, 2025, 12:34pm UTC](https://discourse.julialang.org/t/silent-type-promotion-when-creating-range-using-ac/127671 "2025-04-03T12:34:10Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![marcsgil](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/marcsgil/32/33908_2.png) [@marcsgil](https://discourse.julialang.org/u/marcsgil)\
**Post date:** [April 3, 2025, 12:34pm UTC](https://discourse.julialang.org/t/silent-type-promotion-when-creating-range-using-ac/127671/1 "2025-04-03T12:34:10Z")

</div>

I was meddling with ranges and found the following strange behaviour: suppose I want to create a range starting at 0, with step of 1 and length 10. If I want to work with `Float32`s, I’d type something like

```julia
r1 = 0.0f0:1.0f0:9.0f0

```

I then tried to use this within some `KernelAbstraction` GPU kernels and I kept getting errors.

Upon further investigation, I saw that

```julia
typeof(r1) = StepRangeLen{Float32, Float64, Float64, Int64}

```

so I got `Float64`s in the the `ref` and `step` fields of [StepRangeLen](https://docs.julialang.org/en/v1/base/math/#Base.StepRangeLen), instead of the expected `Float32`.

If I explicitly define

```julia
r2 = StepRangeLen(0f0, 1f0, 10)

```

I get the expected

```julia
typeof(r2) = StepRangeLen{Float32, Float32, Float32, Int64}

```

and the errors I got from the GPU kernel go away.

My question is if this silent type promotion is intended, and, if so, what is the reason behind it.

---

<div class="post-metadata">

**Author:** ![mikmoore](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mikmoore/32/31109_2.png) [@mikmoore](https://discourse.julialang.org/u/mikmoore)\
**Post date:** [April 3, 2025, 2:23pm UTC](https://discourse.julialang.org/t/silent-type-promotion-when-creating-range-using-ac/127671/2 "2025-04-03T14:23:12Z")

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What’s happening is more explicit with `Float64`:

```julia-repl
julia> typeof(0:0.1:1)
StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}

```

It’s a design choice, and certainly a debatable one at that. It’s done to avoid roundoff issues. Compare

```julia
julia> collect(0:0.1:1)
11-element Vector{Float64}:
 0.0
 0.1
 0.2
 0.3
 0.4
 0.5
 0.6
 0.7
 0.8
 0.9
 1.0

julia> collect(StepRangeLen(0,0.1,11))
11-element Vector{Float64}:
 0.0
 0.1
 0.2
 0.30000000000000004
 0.4
 0.5
 0.6000000000000001
 0.7000000000000001
 0.8
 0.9
 1.0

```

This issue _appears_ cosmetic, but it can actually lead to significant ambiguity. Knowing that `0.1*3 == 0.30000000000000004 > 0.3`, should `last(0.0:0.1:0.3)` be `2.0`, `3.0`, or `0.30000000000000004`? The `TwicePrecision` step is the only way to give the user what they (usually) want while maintaining consistency.

But I will sympathize that I, too, find it a tedious to have to use the `StepRangeLen` constructor when I’m okay with a lower precision step. Especially because there is a [performance cost](https://discourse.julialang.org/t/confusion-on-performance-when-using-the-broadcasting-macro-vs-explicit-operators/127380/7).

---

<div class="post-metadata">

**Author:** ![mcabbott](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mcabbott/32/6603_2.png) [@mcabbott](https://discourse.julialang.org/u/mcabbott)\
**Post date:** [April 3, 2025, 2:41pm UTC](https://discourse.julialang.org/t/silent-type-promotion-when-creating-range-using-ac/127671/3 "2025-04-03T14:41:09Z")

</div>

You might be looking for `LinRange(0f0, 9f0, 10)`, which is the low-tech cousin of `StepRangeLen`, with none of this higher-precision-intermediate cleverness.