let eps = 1e-10

fn diff(f: Fn[(Scalar) -> Scalar], x: Scalar) -> Scalar =
    (f(x + eps) - f(x)) / eps

assert_eq(diff(log, 2.0), 1 / 2, 1e-5)
assert_eq(diff(sin, 0.0), 1.0, 1e-5)

fn f(x: Scalar) -> Scalar = x * x + 4 * x + 1

assert_eq(diff(f, 2.0), 8.0, 1e-5)

# TODO: It would be so cool if we could write it in a generic way:
#
# fn diff<X, Y>(f: Fn[(X) -> Y], x: X) -> Y / X =
#   (f(x + eps · unit_of(x)) - f(x)) / (eps · unit_of(x))