Creating an abstract type as a wrapper for types different functions that serve the same purpose

General Usage
,
2

You can either do

struct Model
    kernel::Kernel
end

or

struct Model{T <: Kernel}
    kernel::T
end

In the first one, Model does not have information in its type about which kernel it stores. This will probably lead to a performance penalty for you, but that can’t be said for sure without knowing the context.
The second one is a parametric type, parameterized by which kernel type it stores. Note that I made the type immutable. I expect that’s probably what you want also, as I don’t expect there is value in a mutable kernel (I could be wrong), and mutability also often comes with a performance penalty.

Although it was not part of the question:

struct ExponentialKernel <: Kernel
    ExponentialKernel(rate::Number, x::Number) = exp(-1*rate*x)
end

I don’t think what you wrote does what you want:

julia> ExponentialKernel(5, 5)
1.3887943864964021e-11
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