# How to avoid type instability in the return type of my function?

**URL:** <https://discourse.julialang.org/t/how-to-avoid-type-instability-in-the-return-type-of-my-function/79291>\
**Category:** General Usage\
**Tags:** dispatch, type-stability\
**Created:** [April 10, 2022, 9:10am UTC](https://discourse.julialang.org/t/how-to-avoid-type-instability-in-the-return-type-of-my-function/79291 "2022-04-10T09:10:03Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![ultrapoci](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ultrapoci/32/25356_2.png) [@ultrapoci](https://discourse.julialang.org/u/ultrapoci)\
**Post date:** [April 10, 2022, 9:10am UTC](https://discourse.julialang.org/t/how-to-avoid-type-instability-in-the-return-type-of-my-function/79291/1 "2022-04-10T09:10:04Z")

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I have a struct and a simple function:

```julia
struct SU2Element <: AbstractMatrix{ComplexF64}
	t₁::ComplexF64
	t₂::ComplexF64

	function SU2Element(t₁::Number, t₂::Number)
		SU2Element(ComplexF64(t₁), ComplexF64(t₂))
	end

	function SU2Element(t₁::ComplexF64, t₂::ComplexF64)
		new(t₁, t₂)
	end
end

```

```julia
function normalizeSU2(S::SU2Element)
	sqrtΔ = √det(S)
	t₁ = S.t₁ / sqrtΔ
	t₂ = S.t₂ / sqrtΔ
	SU2Element(t₁, t₂)
end

```

This basically takes an `SU2Element <: AbstractMatrix{Complex}` and normalizes it to have determinant 1.  
In an external function, I need to construct an `SU2Element` and normalize it, but I also need `sqrtΔ` before the normalization. Since I think it is pointless to calculate `sqrtΔ` in `normalizeSU2` just to have it dropped and recalculated outside, I though of returning `sqrtΔ` from `normalizeSU2` in a tuple, like this:

```julia
function normalizeSU2(S::SU2Element, returndet = false)
	sqrtΔ = √det(S)
	t₁ = S.t₁ / sqrtΔ
	t₂ = S.t₂ / sqrtΔ
	if returndet
		SU2Element(t₁, t₂), sqrtΔ
	else
		SU2Element(t₁, t₂)
	end
end

```

The problem with this approach is that it is not type stable. I know I shouldn’t optimize prematurely, but this function gets called a lot in a hot loop, and having already recognized the type instability, I though of trying to solve it.

One solution is to always return `SU2Element(t₁, t₂), sqrtΔ` and just not use `sqrtΔ` in places I don’t need it. But this seems a bit off, I would prefer for the function to return `sqrtΔ` when I’m explicit about it.

Another approach is to just have two different functions with different names, which works and it is explicit, but also knowing how powerful Julia is, I would like to at least try to find a more Julian solution.

Finally, I could pass `Val(true)` or `Val(false)` and dispatch on that, but that would require me to pass `Val(false)` every time I don’t need `sqrtΔ` which is a bit annoying.

Is there a nice way of solving this, that lets me call `normalizeSU2(S)` and have only `SU2Element` returned, and be explicit when I need `sqrtΔ`?

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**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [April 10, 2022, 9:33am UTC](https://discourse.julialang.org/t/how-to-avoid-type-instability-in-the-return-type-of-my-function/79291/2 "2022-04-10T09:33:59Z")

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> [@ultrapoci](#):
>
> Another approach is to just have two different functions with different names, which works and it is explicit,

I would go for this. (Or just write `a, _ = f(...)` to ignore the second return value)

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**Author:** ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)\
**Post date:** [April 10, 2022, 2:34pm UTC](https://discourse.julialang.org/t/how-to-avoid-type-instability-in-the-return-type-of-my-function/79291/3 "2022-04-10T14:34:41Z")

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what about having 2 methods of the function, where the second one lets the caller pass in the sqrt?

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**Author:** ![torrance](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/torrance/32/38990_2.png) [@torrance](https://discourse.julialang.org/u/torrance)\
**Post date:** [April 11, 2022, 4:35am UTC](https://discourse.julialang.org/t/how-to-avoid-type-instability-in-the-return-type-of-my-function/79291/4 "2022-04-11T04:35:59Z")

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Yeah, it’s a little awkward.

I would probably always return the determinant, and just let the caller throw it away if needs be, ie. `su2, _ = normlizeSU2(su2)`. Seems simple enough.