NetPromoterScore.jl is a simple package to calculate NPS.
julia -e 'using Pkg; Pkg.add("NetPromoterScore")'using NetPromoterScore
nps(0) # NetPromoterScore.detractor
nps([0, 7, 9]) # 0.0
nps([NetPromoterScore.passive, NetPromoterScore.promoter]) # 50.0