the same example as s3inheritance.R in julia

ricker.jl

using Random, Distributions, Plots

# the parent type
abstract type ricker end

# derived, concrete types
mutable struct ricker_deterministic <: ricker
    x::Float64
    r::Float64
    k::Float64
end

mutable struct ricker_stochastic <: ricker
    x::Float64
    r::Float64
    k::Float64
    sd::Float64
end

function setup_ricker(x0, r, k, sd = nothing)
    if isnothing(sd)
        return ricker_deterministic(x0, r, k)
    else
        return ricker_stochastic(x0, r, k, sd)
    end
end

function output(model::ricker)
    return model.x
end

# step will dispatch correctly on the concrete type
function step(model::ricker_stochastic)
    theta = rand(Normal(0, model.sd))
    model.x = model.x * (model.r - model.r * (model.x / model.k)) * exp(theta)
end

function step(model::ricker_deterministic)
    model.x = model.x * (model.r - model.r * (model.x / model.k))
end

t = 100
x0 = 1
k = 20
sd = 0.1
r = 2.2

function draw_ricker_traj(t, x0, k, r, sd = nothing)
    mod = setup_ricker(x0, r, k ,sd)
    out = Vector{Float64}(undef, t)
    out[1] = output(mod)
    for i = 2:t
        step(mod)
        out[i] = output(mod)
    end
    return out
end

out_d = draw_ricker_traj(t, x0, k, r)
out_s = map(x -> draw_ricker_traj(t, x0, k, r, sd), 1:100)

plot(1:100, out_s, linealpha = 0.1, linecolor = "blue", legend = false)
plot!(1:100, out_d, color = "black", linewidth = 2)