# Why isn't this true: `[mean, mean, mean] isa Vector{Function}`? but this is: `[mean, mean, sum] isa Vector{Function}`?

Why isn’t this true: `[mean, mean, mean] isa Vector{Function}`? but this is: `[mean, mean, sum] isa Vector{Function}`?

Basically I am writing a function like this, but it’s not dispatching on `applyfns2tuples([mean, mean,mean],[rand(5), rand(5), rand(5)])`.

``````function applyfns2tuples(fns::Vector{Functions}, tuples)
((fns1(tuples1) for (fns1,tuples1) in zip(fns, tuples)...)
end
``````

Type covariance. `Vector{Float64}` is not a `Vector{Number}`. `mean` is a function and every function is a type, so `Vector{#mean}` where `#mean <: Function`. Same thing as the number case.

If you want, you can do `Function[mean, mean, mean]` to get `Vector{Function}`.

For another option, see this package which really, really, really should be in `Base`.

This is the same phenomenon as

``````julia> Vector{Int} <: Vector{Number}
false
``````

even though

``````julia> Int <: Number
true
``````

I believe it has to do with type covariance.

On the other hand

``````julia> Vector{Int} <: Vector{T} where {T<:Number}
true
``````

``````julia> [mean, mean, mean] isa Vector{T} where {T<:Function}
true
``````

Or use shorthand notation

``````julia> [mean, mean, mean] isa Vector{<:Function}
true
``````
2 Likes

It makes sense now. That’s why Julia can specialise on functions where functions are passed as arguments

4 Likes

If you need to dispatch you can use `Vector{<:Function}` which encompasses all the vectors of something that is a subtype of `Function`

This will work, and simplifies your notation a bit (drop the splatting):

``````apply(funs::Vector{<:Function}, args) = (f(arg) for (f, arg) in zip(funs, args))
``````

I played around and tried to use tuples instead of `Vector{<:Function}`, but could not find a way to express it. For example,

``````julia> [mean, mean, sum] isa Vector{<:Function}
true
``````

while

``````julia> (mean, mean, sum) isa NTuple{3, <:Function}
false
``````
1 Like
``````julia> (mean, mean, sum) isa NTuple{3, Function}
true

julia> NTuple{3, <:Function}
Tuple{#s1,#s1,#s1} where #s1<:Function
``````
2 Likes