Arithmetic-geometric mean

func agm(a, g) {
    loop {
        var (a1, g1) = ((a+g)/2, sqrt(a*g))
        [a1,g1] == [a,g] && return a
        (a, g) = (a1, g1)
    }
}

say agm(1, 1/sqrt(2))

Output:

0.847213084793979086606499123482191636481445910327