AFAIU, it's not only very large or small numbers that are affected, but any rational number with a denominator larger than 2*64. For most applications it's completely unpredictable when the switch to Num happens.
Nearly, the docs say "To prevent the numerator and denominator from becoming pathologically large, the denominator is limited to 64 bit storage." (https://docs.raku.org/type/Rat)
Please bear in mind that in raku (like perl and other untyped scripting languages) it is normal to say:
my $x = 1;
say 1 + $x; #2
say 'a' ~ $x; #'a1'
My point is that when the type is automatically inferred/coerced like this, is is very natural that small/whole Numerics are Int, that medium/fractional/decimal Numerics are Rat and that large/exponential Numerics are Num (ie double). And that you can freely perform maths operations mixing the various Numeric types at will.
my $y = 1;
say 1 + $y; #2 Int
say 1.1 + $y; #2.1 Rat
say 1e27 + $y; #1e27 Num
And, in raku, if you want to control the type, then just use the type system like this.
my FatRat $r;
I also think from a 2nd order point of view that a denominator of 2*64 means you are dealing with quite a small number 5e-20 ish), although admittedly that is a matter of taste and machine performance. It makes most sense in my view to go with Rat (which is stored as an Int (uint64) for the numerator and an Int for the numerator.
That way (i) you get to use all those transistors that you bought for your FPU and (ii) you do not get a surprise as Rat operations perform slowly without warning.
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And yes - @lizmat has pointed out the various pragmas to let you control the behaviour you want if you disagree.
