How to make a Currency class from std.BigInt?

[RuZzz via Digitalmars-d-learn]

With "eris" lib some problems, the first error:
.../.dub/packages/eris-0.0.1/eris/integer/digits.d(241): Error: 
cannot implicitly convert expression (digits.length) of type 
ulong to int

What can I import to use rational numbers?
I found it
https://github.com/dsimcha/Rational/blob/master/rational.d
https://github.com/d-gamedev-team/gfm/blob/master/math/gfm/math/rational.d
but I do not understand what the story ended with rational 
numbers in D.