The goal of numint is to ...
This is a basic example which shows you how to solve a common problem:
library(numint)
set.seed(1231)
ans <- numint(dbeta, a = 0, b = .5, shape1 = 2, shape2 = 20)
ans
#> MONTE CARLO INTEGRATION
#> N: 100
#> Volume: 0.5000
#> 1.4385 +- 1.4996
plot(ans)