Add test file for the 'modf' routine in rakudo

[tbrowder]

See rakudo PR rakudo/rakudo#4434.

[tbrowder]

Note there will be more tests added.

[vrurg]

I wonder if there're any chance for errors on user side. I'd be glad to see testing for incorrect user input. How modf must react on *? Inf, NaN?

Also, I didn't follow the original discussion about modf, but why $val is in stringified form only? This way you test the Cool candidate only. This is right, in the light of the proposed PR you kinda test every multi-candidate then. But testing must not rely on particular implementation.

And then, tests for positives must be explicitl, i.e. included into the table. When you use abs you turn $val into a Numeric, thus testing different multi-candidates compared to the tests for negative. It effectively means that you kind of testing two things at once: multi-dispatch and the type of argument.

[tbrowder]

Note I haved closed the associated Rakudo PR #4434 which I will be rewriting and resubmitting as a new PR. This roast PR will continue to be used by the new Rakudo PR (which I will not push until I am happy with it).

[tbrowder]

@vrurg, I've been doing some experiments and would appreciate your thoughts on this:

• use only one set of desired test values, all as strings with a leading '-' sign:

  my $x-neg-str = '-0.e5';

• remove the leading minus signs from the strings to get positive versions:

  my $x-pos-str = $x-neg-str.substr: 1;

• convert each to a RatStr or NumStr by using <<>> quotes:

  my $x-neg-allomorph = <<$x-neg-str>>;
  my $x-pos-allomorph = <<$x-pos-str>>;

• convert each allomorph to the appropriate number type:

  my ($x-neg-rat, $x-neg-num, $x-pos-rat, $x-pod-num);
  if $x-neg-allomorph ~~ Rat {
  $x-neg-rat = $x-neg-allomorph.Rat;
  $x-pos-rat = $x-pos-allomorph.Rat;
  }
  elsif $x-neg-allomorph ~~ Num {
  $x-neg-num = $x-neg-allomorph.Num;
  $x-pos-num = $x-pos-allomorph.Num;
  }
  else {
  die "FATAL: Unexpected input '$x-neg-str' of type '{$x-neg-str.^name}'";
  }

Then I think we would have all the number type possibilities to test, as applicable.