Add UnknownDirection

src/QEDbase.jl

@@ -37,7 +37,7 @@ export mass, charge
 export base_state, propagator
 
 # directions
-export ParticleDirection, Incoming, Outgoing
+export ParticleDirection, Incoming, Outgoing, UnknownDirection
 export is_incoming, is_outgoing
 
 # polarizations and spins

src/particles/direction.jl

@@ -69,3 +69,29 @@ struct Outgoing <: ParticleDirection end
 is_incoming(::Outgoing) = false
 is_outgoing(::Outgoing) = true
 Base.show(io::IO, ::Outgoing) = print(io, "outgoing")
+
+"""
+    UnknownDirection <: ParticleDirection
+
+Concrete implementation of a [`ParticleDirection`](@ref) to indicate that a particle has an unknown direction.
+This can mean that a specific direction does not make sense in the given context, that the direction is unavailable, or that it is unnecessary.
+
+```jldoctest
+julia> using QEDbase
+
+julia> UnknownDirection()
+unknown direction
+```
+
+!!! note "ParticleDirection Interface"
+    Besides being a subtype of [`ParticleDirection`](@ref), `UnknownDirection` has
+
+    ```julia
+    is_incoming(::UnknownDirection) = false
+    is_outgoing(::UnknownDirection) = false
+    ```
+"""
+struct UnknownDirection <: ParticleDirection end
+is_incoming(::UnknownDirection) = false
+is_outgoing(::UnknownDirection) = false
+Base.show(io::IO, ::UnknownDirection) = print(io, "unknown direction")

test/particle_properties.jl

@@ -9,7 +9,7 @@ test_broadcast(x::AbstractSpinOrPolarization) = x
 
 @testset "scalar broadcasting" begin
     @testset "directions" begin
-        @testset "$dir" for dir in (Incoming(), Outgoing())
+        @testset "$dir" for dir in (Incoming(), Outgoing(), UnknownDirection())
             @test test_broadcast.(dir) == dir
         end
     end