I'm not sure this is such a good idea. I love rational datatype, but it's too easy to shoot yourself in the foot with simple numerical procedures resulting in gigantic bignum denominators.
Rational numbers are horrible for most numerical computing. The denominators double in length with every multiplication, so computation speed gets slow exponentially.
Any time there’s any degree of uncertainty about a quantity (e.g. it comes from a physical measurement) there’s also no longer any advantage to using rational arithmetic. This turns out to encompass most practical situations.
Rational arithmetic also breaks down entirely in the face of square roots or trig functions, unless you go for a fully symbolic computation environment, which gets even much slower.
Rational arithmetic is mostly nice when the problems have been carefully chosen so the operations will stay rational and the answers will work out nicely, e.g. in high school homework.
default doesn't mean only, from memory Rakudo smooshes Rats into floating point Nums when the denominator gets above 2 to the 64th or so.
FatRat are an infinite precision rat. FatRats, like RATs are Cool (which is a double entendre, because Cool means it knows how to be a string, which is apparently "Cool") The default recently has not been fatrat but one of the many benefits of a language under development for 20 years is that quite possibly that default was changed last weekend. Or maybe fatrat were default for a painful week back in '02. Probably not, but it could have happened.
Perl does the right thing but if you're really unhappy about not using floats you can force a .num conversion into floating point or you could express a number in scientific notation to force floats. Perl really wants to make a division problem into a Rat unless you work hard to stop it.
In general, for the past 30 years or so, if you're doing something weird, Perl will work and probably give faster and possibly more accurate results than most other tools, but the rest of the world will not be interested in debugging perl6 but will instead be asking "why you use perl6 instead of Gnu-R or mathematica or ?" or whatever is trendy momentarily today. Perl will never be trendy yet the whole world runs on it.
I'm not sure this is such a good idea. I love rational datatype, but it's too easy to shoot yourself in the foot with simple numerical procedures resulting in gigantic bignum denominators.