# Restricting method dispatch to a few concrete types
**URL:** https://discourse.julialang.org/t/restricting-method-dispatch-to-a-few-concrete-types/63765
**Category:** New to Julia
**Created:** [June 29, 2021, 3:26pm UTC](https://discourse.julialang.org/t/restricting-method-dispatch-to-a-few-concrete-types/63765 "2021-06-29T15:26:39Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@lrnv](https://discourse.julialang.org/u/lrnv)
#### Post date: [June 29, 2021, 3:26pm UTC](https://discourse.julialang.org/t/restricting-method-dispatch-to-a-few-concrete-types/63765/1 "2021-06-29T15:26:39Z")
Hey,
I have a method :
```julia
function f(x)
return x
end
```
for which i have a faster version that works only if x is either of type T1 or of type T2. I tried writting:
```julia
function f(x::T) where {Union{T1,T2} <: T <: Union{T1,T2}}
//stuff
end
```
but it did not work.
How can i specify that the Type must be either T1 or T2 ? (it cannot be any subtypes of T1 and T2, which are already concretes types).
The specific usecase is `T1,T2 = Float32,Float64`, for wich the function directly calls into lapack.
---
### Author:  [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)
#### Post date: [June 29, 2021, 3:38pm UTC](https://discourse.julialang.org/t/restricting-method-dispatch-to-a-few-concrete-types/63765/2 "2021-06-29T15:38:48Z")
You just want ` where T <: Union{T1,T2}`. Concrete types can’t have subclasses.
---
### Author:  [@lrnv](https://discourse.julialang.org/u/lrnv)
#### Post date: [June 29, 2021, 3:39pm UTC](https://discourse.julialang.org/t/restricting-method-dispatch-to-a-few-concrete-types/63765/3 "2021-06-29T15:39:59Z")
Aaand you are right. Thanks
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [June 29, 2021, 3:46pm UTC](https://discourse.julialang.org/t/restricting-method-dispatch-to-a-few-concrete-types/63765/4 "2021-06-29T15:46:32Z")
Or just
```julia
function f(x::Union{T1,T2})
# stuff
end
```
---
### Author:  [@lrnv](https://discourse.julialang.org/u/lrnv)
#### Post date: [June 29, 2021, 3:50pm UTC](https://discourse.julialang.org/t/restricting-method-dispatch-to-a-few-concrete-types/63765/5 "2021-06-29T15:50:26Z")
Yep but does not work in my case since many parameters use the `T` parametric type, and I want it to be the same for everyone.
Just to note that :
`function f(x::Union{T1,T2}, y::Union{T1,T2}) where {T1,T2}`
is not the same as
`function f(x::T, y::T) where {T<:Union{T1,T2}}`.
In the first case `x` and `y` might not be the same type, and in the second they are guarenteed to be the same type.
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [June 29, 2021, 4:04pm UTC](https://discourse.julialang.org/t/restricting-method-dispatch-to-a-few-concrete-types/63765/6 "2021-06-29T16:04:34Z")
Yes. It just wasn’t necessary in your particular example, so I wanted to point it out.