How to tell Julia that my type is not iterable?

How do I tell Julia that my object is not iterable? How do I make a, b, c = some_function.(mytype, (1, 2, 3)) work automatically without throwing no method matching length(::MyType)?

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the easy way:

wrap it in something iterable

some_function.(Ref(mytype), (1, 2, 3))

if you want this to work in general without wrapping would be to change the BroadcastStyle of your object

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You want Base.broadcastable(x::T) = Ref(x). See here

julia> struct A end

julia> foo(a::A, x) = x;

julia> foo.(A(), [1, 2])
ERROR: MethodError: no method matching length(::A)

julia> Base.broadcastable(a::A) = Ref(a);

julia> foo.(A(), [1, 2])
2-element Vector{Int64}: