in reply to Re^5: Small Perl 6 discoveries II, Rats
in thread [Perl6] Small discoveries I, __DATA__

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.

  • Comment on Re^6: Small Perl 6 discoveries II, Rats

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Re^7: Small Perl 6 discoveries II, Rats
by syphilis (Archbishop) on Oct 22, 2017 at 23:12 UTC
    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]
        That's a mutable "container" aka a variable (a Scalar in this case) containing one immutable value

        I really do not understand the use of the term "immutable".
        Is it perhaps meant to be some shorthand method of signifying that the numerator and denominator are co-prime ? (For this, the gmp C library documentation uses the term "canonical" - in the sense, I think, of "authoritative, standard, accepted".)
        Admittedly, the connection between the meanings of "co-prime" and "immutable" seems very tenuous to me, but it's about all that I can come up with.

        Cheers,
        Rob
[reply]
        You can give $x a type, but prepare to be disappointed. 
        > my Rat $x = 0.1;
> $x *= 0.1.FatRat;
Type check failed in assignment to $x; expected Rat but got FatRat (Fa
+tRat.new(1, 100))

        [download]
[reply]
[d/l]

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