Issue #8850 has been updated by Kenta Murata.


I've updated the patch by rebasing and fixing syntax.
https://github.com/mrkn/ruby/compare/64634f92f3474babab93264b3c7557043f6753e7...71e463b80d17959f3a4cfd49d7773818fa671037

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Feature #8850: Convert Rational to decimal string
https://bugs.ruby-lang.org/issues/8850#change-45223

* Author: Yui NARUSE
* Status: Assigned
* Priority: Normal
* Assignee: Yukihiro Matsumoto
* Category: 
* Target version: next minor
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On Ruby 2.1.0, decimal literal is introduced.
It generates Rational but it cannot easily convert to decimal string.
You know, Rational#to_f is related to this but
* Float is not exact number
** 0.123456789123456789r.to_f.to_s #=> "0.12345678912345678"
* it can't handle recursive decimal
** (151/13r).to_f.to_s #=> "11.615384615384615"

* the method name
** to_decimal
** to_decimal_string
** to_s(format: :decimal)
** extend sprintf
* how does it express recursive decimal
** (151/13r).to_decimal_string #=> "11.615384..."
** (151/13r).to_decimal_string #=> "11.(615384)"

Example implementation is following.
Its result is
** 0.123456789123456789r.to_f.to_s #=> "0.123456789123456789"
** (151/13r).to_f.to_s #=> "11.(615384)"

```
class Rational
  def to_decimal_string(base=10)
    n = numerator
    d = denominator
    r, n = n.divmod d
    str = r.to_s(base)
    return str if n == 0
    h = {}
    str << '.'
    n *= base
    str.size.upto(Float::INFINITY) do |i|
      r, n = n.divmod d
      if n == 0
        str << r.to_s(base)
        break
      elsif h.key? n
        str[h[n], 0] = '('
        str << ')'
        break
      else
        str << r.to_s(base)
        h[n] = i
        n *= base
      end
    end
    str
  end
end
```



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