Using numbers to do arithmetic with fractions in PureScript may yield surprising results:
> 0.1 + 0.2
0.30000000000000004
The same can be expressed accurately with Rational using the (%) operator:
> import Data.Rational
> (1 % 10) + (2 % 10)
3 % 10
You can turn a Rational to a Number:
> toNumber (3 % 10)
0.3
bower install purescript-rationals
Module documentation is published on Pursuit.