How to calculate a constant the first time a function is called?

General Usage
7

If your function has some input, you might better avoid at all this global state, and wrap the constant precomputed in the input, such that the function is completely self-contained. Something like:

julia> struct MyInput
           x::Vector{Float64}
           constant::Float64
       end

julia> MyInput(x::Vector{Float64}) = MyInput(x, length(x)*sum(x)) # invented a calculation for the constant
MyInput

julia> input = MyInput(rand(3)) # here the constant is computed
MyInput([0.607776859483833, 0.630463715553019, 0.01606684134650216], 3.762922249150063)

julia> function foo!(input)
           (;x, constant) = input
           x .= constant .* x  # mutates the `x` entry of input
       end
foo (generic function with 1 method)

julia> foo!(input)
MyInput([2.2870170670702663, 2.372385942536272, 0.06045827477631713], 3.762922249150063)

julia> foo!(input)
MyInput([8.605867405864627, 8.92710384674058, 0.22749978730103176], 3.762922249150063)
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