# How to dispatch on a union of parametric types with partial Type-instruction?

**URL:** <https://discourse.julialang.org/t/how-to-dispatch-on-a-union-of-parametric-types-with-partial-type-instruction/9165>\
**Category:** New to Julia\
**Created:** [February 19, 2018, 3:19pm UTC](https://discourse.julialang.org/t/how-to-dispatch-on-a-union-of-parametric-types-with-partial-type-instruction/9165 "2018-02-19T15:19:39Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Datseris](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/datseris/32/13406_2.png) [@Datseris](https://discourse.julialang.org/u/Datseris)\
**Post date:** [February 19, 2018, 3:19pm UTC](https://discourse.julialang.org/t/how-to-dispatch-on-a-union-of-parametric-types-with-partial-type-instruction/9165/1 "2018-02-19T15:19:39Z")

</div>

I have two types, `MDI` and `ODEIntegrator`. Both are parameterized with _many_ types. However, for my dispatch purposes, I only care about the first of `MDI` and the second of `ODEIntegrator`. I can dispatch on them like:

```julia
# Version 1
test(integ::ODEIntegrator{Alg, S}) where{Alg, S<:SMatrix} = 1
test(integ::MDI{false}) = 1

# Version 2
test(integ::ODEIntegrator{Alg, S}, ws_index) where{Alg, S<:Matrix} = 2
test(integ::MDI{true}) = 2

```

The above works perfectly fine.

But now, I am really believing that I can achieve the same result with only 2 function definitions, by using `Union`. Unfortunately I think I have not understood how to use it, because I though that this:

```julia
function test(
    integ::Union{A, B}) where
    {A<:ODEIntegrator{Alg, S} where {Alg, S<:SMatrix}, B<:MDI{false}}
  1
end

function test(
    integ::Union{A, B}) where
    {A<:ODEIntegrator{Alg, S} where {Alg, S<:Matrix}, B<:MDI{true}}
  2
end

```

should be equivalent with the other definitions. However, when I try to call the method (_without_ having defined the ones in the start of my post), Julia says that it cannot find any method that matches `test` with my object. Why?

---

<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [February 19, 2018, 4:10pm UTC](https://discourse.julialang.org/t/how-to-dispatch-on-a-union-of-parametric-types-with-partial-type-instruction/9165/2 "2018-02-19T16:10:02Z")

</div>

The current dispatch behavior in Julia v0.6 (and previous versions) requires that _every_ parameter in a method signature must be given a value for the method to match. In your case, that means that whatever types you pass in must allow Julia to assign both the `S` and `B` parameters in the Union. Neither of your types does so (of course) so neither matches the signature.

Fortunately, this requirement is [dropped in Julia v0.7](https://github.com/JuliaLang/julia/pull/23117), so I think your code should just work. Here’s a simpler example:

## v0.6

```julia-auto
julia> struct Foo{A}
       end

julia> struct Bar{B}
       end

julia> f(::Union{Foo{A}, Bar{B}}) where {A, B} = 1
f (generic function with 1 method)

julia> f(Foo{Int}())
ERROR: MethodError: no method matching f(::Foo{Int64})
Closest candidates are:
  f(::Union{Bar{B}, Foo{A}}) where {A, B} at REPL[3]:1

```

## v0.7

```julia-auto
julia> struct Foo{A}
       end

julia> struct Bar{B}
       end

julia> f(::Union{Foo{A}, Bar{B}}) where {A, B} = 1
f (generic function with 1 method)

julia> f(Foo{Int}())
1

```