# Warning for unused type variables in where
**URL:** https://discourse.julialang.org/t/warning-for-unused-type-variables-in-where/30360
**Category:** Internals & Design
**Tags:** question
**Created:** [October 27, 2019, 9:47am UTC](https://discourse.julialang.org/t/warning-for-unused-type-variables-in-where/30360 "2019-10-27T09:47:47Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [October 27, 2019, 9:47am UTC](https://discourse.julialang.org/t/warning-for-unused-type-variables-in-where/30360/1 "2019-10-27T09:47:47Z")
I am wondering whether it would make sense to warn unused extra type variables for cases like
```julia
foo(a::AbstractVector{T}) where {T,S} = 1
```
where it is very likely that `S` is a typo.
It is certainly innocuous as long as it is not used (and then one gets an error), this would just be a convenience feature.
(asking here because I could not find an issue about this)
---
### Author:  [@yuyichao](https://discourse.julialang.org/u/yuyichao)
#### Post date: [October 27, 2019, 1:08pm UTC](https://discourse.julialang.org/t/warning-for-unused-type-variables-in-where/30360/2 "2019-10-27T13:08:29Z")
Like all other similar features, it should either
1. Off by default, turn on with cmdline arg or env, or
2. Have a way to turn it off in the code, or
3. Be an error
As of now, an external tool like linter is a much better place for it.
---
### Author:  [@tkf](https://discourse.julialang.org/u/tkf)
#### Post date: [October 29, 2019, 1:56am UTC](https://discourse.julialang.org/t/warning-for-unused-type-variables-in-where/30360/3 "2019-10-29T01:56:03Z")
How about using `Test.detect_unbound_args`?
```julia
julia> module Demo
bad(a::AbstractVector{T}) where {T,S} = 1
good(a::AbstractVector{T}) where {T} = 1
end
Main.Demo
julia> detect_unbound_args(Demo)
[1] bad(a::AbstractArray{T,1}) where {T, S} in Main.Demo at REPL[121]:2
```
FYI I wrote a convenient wrapper for this and similar automatable checks: [https://github.com/tkf/Aqua.jl](https://github.com/tkf/Aqua.jl) (required to workaround [https://github.com/JuliaLang/julia/pull/31972](https://github.com/JuliaLang/julia/pull/31972))
---
### Author:  [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [October 29, 2019, 7:21am UTC](https://discourse.julialang.org/t/warning-for-unused-type-variables-in-where/30360/4 "2019-10-29T07:21:11Z")
That’s very helpful, thanks.
From this discussion, I realized that functionality like this should live in a linter, and user who want this are best served by making linting automated and easy to use, possibly as part of CI. You package does exactly this.