# Why type instability?
**URL:** https://discourse.julialang.org/t/why-type-instability/4013
**Category:** General Usage
**Tags:** question
**Created:** [May 31, 2017, 12:56pm UTC](https://discourse.julialang.org/t/why-type-instability/4013 "2017-05-31T12:56:57Z")
**Posts on this page:** 5
**Page:** 2
### Author:  [@e3c6](https://discourse.julialang.org/u/e3c6)
#### Post date: [November 23, 2017, 11:08am UTC](https://discourse.julialang.org/t/why-type-instability/4013/21 "2017-11-23T11:08:59Z")
I would like a feature like this as well… Like an assert. @type\_stable\_assert 😛
---
### Author:  [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)
#### Post date: [November 23, 2017, 12:33pm UTC](https://discourse.julialang.org/t/why-type-instability/4013/22 "2017-11-23T12:33:27Z")
The hard part is finding a set of static rules for a subset of Julia, but it does seem like it might be possible to find such a rule set and let people declare sections of code to follow those rules and accordingly get guaranteed tight type bounds and compile-time type checking. Definitely an interesting area for future work.
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### Author:  [@e3c6](https://discourse.julialang.org/u/e3c6)
#### Post date: [November 23, 2017, 12:50pm UTC](https://discourse.julialang.org/t/why-type-instability/4013/23 "2017-11-23T12:50:59Z")
On the other hand, you can type assert return types, `fun(x)::Int`, which automatically makes the function type stable. This is the way C guarantees type stability.
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### Author:  [@mauro3](https://discourse.julialang.org/u/mauro3)
#### Post date: [November 23, 2017, 1:26pm UTC](https://discourse.julialang.org/t/why-type-instability/4013/24 "2017-11-23T13:26:11Z")
> [@e3c6](#):
>
> you can type assert return types, fun(x)::Int
If you mean that this
```julia
function fun(x)::Int
...
end
```
asserts the return-type, then you’re wrong (otherwise ignore me!). It converts the type:
```julia
julia> function f(x)::Int
x
end
f (generic function with 1 method)
julia> f(Int32(3))
3
julia> Int32(3)::Int
ERROR: TypeError: typeassert: expected Int64, got Int32
```
---
### Author:  [@e3c6](https://discourse.julialang.org/u/e3c6)
#### Post date: [November 23, 2017, 1:36pm UTC](https://discourse.julialang.org/t/why-type-instability/4013/25 "2017-11-23T13:36:08Z")
My point is that it guarantees type-stability. Wherever you call `fun` you are sure the return type is an Int. Also, it only converts the type if it is “convertable”. `fun("hello")` throws an error.
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