# Generic way to dispatch on type of Union type

**URL:** <https://discourse.julialang.org/t/generic-way-to-dispatch-on-type-of-union-type/82745>\
**Category:** General Usage\
**Created:** [June 14, 2022, 1:12pm UTC](https://discourse.julialang.org/t/generic-way-to-dispatch-on-type-of-union-type/82745 "2022-06-14T13:12:25Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![manuelbb-upb](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/manuelbb-upb/32/25222_2.png) [@manuelbb-upb](https://discourse.julialang.org/u/manuelbb-upb)\
**Post date:** [June 14, 2022, 1:12pm UTC](https://discourse.julialang.org/t/generic-way-to-dispatch-on-type-of-union-type/82745/1 "2022-06-14T13:12:25Z")

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Hi,  
I tried to write methods for a function (taking types as arguments) that should dispatch only if one argument supertypes the other, e.g.,

```julia
a_supertypes_b( a::Type, b::Type ) = "no"
a_supertypes_b( ::Type{SUP}, ::Type{SUB} ) where {SUP, SUB<:SUP} = "yes"

```

Of course, my use case was not checking the type relation, but the method signatures looked similar.  
Now, I also wanted a custom method for the case that the supertype `SUP` was a Union type.  
Is there a generic way to dispatch on “types of Union types”?  
First thought:

```julia
a_supertypes_b( ::Type{SUP}, ::Type{SUB} ) where {SUP<:Union, SUB<:SUP} = "yes_for_union"

```

This does not get called:

```julia
T32 = Union{Int32, Float32}
a_supertypes_b( T32, Int32 ) == "yes" # not "yes_for_union"

```

I think, this is because `T32 isa Type{<:Union}` is `false`.  
In fact, for any Union type, `typeof` returns simply `Union`.  
This is also why

```julia
a_supertypes_b( :: SUP, :: Type{SUB} ) where {SUP<:Union, SUB<:SUP} = "yes_for_union"

```

won’t get called for `(T32, Int32)`: Then `SUP == Union` and it does not hold that `Int32 <: Union`.

In all of the above examples, the method for union types does not get called, because `typeof(T32) != Type{T32}` which would discourage from using the dispatch mechanism for types of union types.  
**However** , I know that I can use handwritten method signatures, e.g.,

```julia
T64 = Union{Int64, Float64}
f( :: Type{Union{Int32, Float32}} ) = 32
f( :: Type{T64} ) = 64
# now this is true:
f(T32) == 32 && f(T64) == 64

```

and

```julia
a_supertypes_b( :: Type{SUP}, SUB ) where {SUP<:T32, SUB<:SUP} = "yes_T32"

```

would get called: `a_supertypes_b( T32, Int32 ) == "yes_T32"`.  
So in the above example, for the union type `T32`, Julia dispatches on `Type{T32}` (and not on `typeof(T32) == Union`).

Can I do this generically, if the exact union types are not known beforehand?  
I guess, one difficulty prohibiting a generalization of the last method definition for arbitrary union types lies in the fact, that there is no defining supertype of union types. That is, there is no `SUP_UNION` such that `T <: SUP_UNION` for any union type `T`, which would allow for

```julia
a_supertypes_b( :: Type{SUP}, SUB ) where {SUP<:SUP_UNION, SUB<:SUP} = "yes_for_union"

```

Or does such a supertype exist?  
Am I overlooking something?  
Thanks in advance!

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<div class="post-metadata">

**Author:** ![Sukera](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Sukera](https://discourse.julialang.org/u/Sukera)\
**Post date:** [June 14, 2022, 1:32pm UTC](https://discourse.julialang.org/t/generic-way-to-dispatch-on-type-of-union-type/82745/2 "2022-06-14T13:32:45Z")

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> [@manuelbb-upb](#):
>
> Is there a generic way to dispatch on “types of Union types”?

No, and to quote Jeff “if you do that, please stop”. There’s a juliacon talk from a few years ago where jeff mentions this exact thing, I’ll link it once I find it again. There’s also an issue for this sort of thing, the keywords are “dispatch on type representation”.

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<div class="post-metadata">

**Author:** ![Sukera](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Sukera](https://discourse.julialang.org/u/Sukera)\
**Post date:** [June 14, 2022, 1:37pm UTC](https://discourse.julialang.org/t/generic-way-to-dispatch-on-type-of-union-type/82745/3 "2022-06-14T13:37:21Z")

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This is the talk:

[![](https://global.discourse-cdn.com/julialang/original/3X/1/d/1d5c5567a3f6004a6c9852f4fb8e89eb97fe67be.jpeg "JuliaCon 2019 | What's Bad About Julia | Jeff Bezanson") ](https://www.youtube.com/watch?v=TPuJsgyu87U)

The part I’m talking about is at 15:15, but the whole section starting at 13:00 is probably best for context.

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<div class="post-metadata">

**Author:** ![manuelbb-upb](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/manuelbb-upb/32/25222_2.png) [@manuelbb-upb](https://discourse.julialang.org/u/manuelbb-upb)\
**Post date:** [June 14, 2022, 3:01pm UTC](https://discourse.julialang.org/t/generic-way-to-dispatch-on-type-of-union-type/82745/4 "2022-06-14T15:01:12Z")

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Ok, thank you very much! I think I am beginning to see the problems.  
Here is a related issue on GitHub: [redesign typeof(::Type) · Issue #29368 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/29368)