in reply to Re^4: Small Perl 6 discoveries II, Rats
in thread [Perl6] Small discoveries I, __DATA__

$x.nude

That enables me to see that 1.111111111111111111111.FatRat and 1.111111111111111111111.Rat have equivalent rational values: 
> my $x = 1.111111111111111111111.Rat; $x.nude
(1111111111111111111111 1000000000000000000000)
> my $y = 1.111111111111111111111.FatRat; $y.nude
(1111111111111111111111 1000000000000000000000)

[download]
and that $x (the Rat) gets handled in a way that I don't really expect: 
> my $r1 = $x * 0.3
0.333333333333333
> $r1 = $x * 0.3.Rat
0.333333333333333
> $r1 = $x * 0.3.Num
0.333333333333333
> $r1 = $x * 3e-1
0.333333333333333
> $r1 = $x * 0.3.FatRat
0.3333333333333333333333

[download]
though the last result is as I expected.
Compare those outputs with: 
> my $r2 = $y * 0.3
0.3333333333333333333333
> my $r2 = $y * 0.3.Rat
0.3333333333333333333333
> my $r2 = $y * 0.3.Num
0.333333333333333
> my $r2 = $y * 3e-1
0.333333333333333
> my $r2 = $y * 0.3.FatRat
0.3333333333333333333333

[download]
which is more in keeping with my expectations.

I'm sure it all makes sense if you know how to look at it from the appropriate angle.

Cheers,
Rob

Replies are listed 'Best First'.
Re^6: Small Perl 6 discoveries II, Rats
by Anonymous Monk on Oct 22, 2017 at 15:14 UTC
    From docs.perl6.org/type/Rat (anons can't post full urls):
    To prevent the numerator and denominator from becoming pathologically large, the denominator is limited to 64 bit storage. On overflow of the denominator a Num (floating-point number) is returned instead.
    However, Rat values generated from decimal notation can violate the 64-bit limit.

    You can use $x.WHAT to find $x's type if you get tired of guessing based on the number of decimal places displayed.

[reply]
      To prevent the numerator and denominator from becoming pathologically large, the denominator is limited to 64 bit storage

      Well, that doesn't seem to be the case with (latest) rakudo-star-2017.07: 
      > my $x=9.999999999999999999999999999999999999999999999.Rat; $x.nude
(9999999999999999999999999999999999999999999999 1000000000000000000000
+000000000000000000000000)
> $x.WHAT
(Rat)

      [download]
      but the base 10 denominator of 1000000000000000000000000000000000000000000000 is a 150-bit value.

      The documentation also concludes with "Rat objects are immutable" - yet the following operation converts $x from a Rat to a FatRat: 
      > $x *= 0.3.FatRat; $x.nude
(29999999999999999999999999999999999999999999997 100000000000000000000
+00000000000000000000000000)
> $x.WHAT
(FatRat)

      [download]
      Am I misunderstanding that documentation ?
      Is there a way of interpreting the documentation such that it does match the above behaviours ?

      Cheers,
      Rob
[reply]
[d/l]
[select]
        the following operation converts $x from a Rat to a FatRat

        That's a a mutable "container" aka a variable (a Scalar in this case) containing one immutable value (a Rat) then another immutable value (a FatRat). 

        # Declare a new identifier $x and bind it to a new mutable Scalar cont
+ainer:
my $x;

# Assign a value into the Scalar:
$x                # An LHS reference to a Scalar returns the Scalar.
   =              # = is assignment. The LHS decides what to do with i
+t.
     9.9;         # Creates a new immutable Rat.

# VAR macro returns what the identifier is bound to:
say $x.VAR.^name; # Scalar -- the type of variable bound to $x.

# All other uses of $x or a Scalar on RHS return the value assigned in
+to the Scalar:
say $x;           # 9.9
say $x.^name;     # Rat -- the type of the value assigned into the Sca
+lar.
$x *= 2;          # This is equivalent to ...
$x = $x * 2;      # ... which assigns result of RHS expression into Sc
+alar on LHS.
say $x;           # 39.6

        [download]

        Hth.

[reply]
[d/l]

In Section Meditations