known = Dict(0=>0, 1=>1)
function fibonacci(n; known = known)
    haskey(known, n) && return known[n]
    known[n] = fibonacci(n - 1) + fibonacci(n - 2)

    return known[n]
end

function fibonacci(n)
    if n ∈ keys(known)
        return known[n]
    end
    res = fibonacci(n-1) + fibonacci(n-2)
    known[n] = res
    return res
end

function get!(t::AbstractDict{K,V}, key::K, default) where K where V
    haskey(t, key) && return t[key]
    t[key] = default

    return t[key]
end

known = Dict(0=>0, 1=>1)
function fibonacci(n; known = known)
    return get!(known, n, fibonacci(n - 1) + fibonacci(n - 2))
end

fibonacci(2)

julia> function foo()
           println("Hello, world!")
           return 10
       end
foo (generic function with 1 method)

julia> known = Dict(0=>0, 1=>1)
Dict{Int64,Int64} with 2 entries:
  0 => 0
  1 => 1

julia> get!(known, 0, foo())
Hello, world!
0