Issue #8850 has been reported by naruse (Yui NARUSE).

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

Author: naruse (Yui NARUSE)
Status: Assigned
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: 
Target version: next minor


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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