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

Rational is just a type alias for Ratio Int and you might want to use Ratio with other than Int. The type you choose must however be an EuclideanRing.

For example, one limitation with Rational is that it can easily overflow the 32-bit PureScript Int. You can get around this problem by using BigInt.

> import Data.Ratio ((%), reduce)
> import Data.BigInt (fromInt, fromString)
> :type fromInt 1 % fromInt 3
Ratio BigInt
> reduce <$> fromString "10" <*> fromString "857981209301293808359384092830482"
(Just fromString "5" % fromString "428990604650646904179692046415241")

bower install purescript-rationals

Module documentation is published on Pursuit.