Reverse polish notation from Ox0dea

rpn_calc.rb

@_=[*?/...?:][(_=$$/$$)..-_]*'';(@__=->__{$_=__[_-_];@_[$_]?$*<<(@_=~/#$_/):(___,$__=$*[-_-_,_+_];$*[-_-_,_+_]=$_[?+]?___+$__:$_[?-]?___-$__:$_[?*]?___*$__:$_[?/]?___/$__:_);__[_]?@__[__[_..-_]]:$*})

# Here we go again! Note - THIS NEEDS REFINEMENT when I have more than a few minutes to look at it
@_ =
  # Range of 0 to 9, joined into a string: '0123456789'
  [*?/...?:][
    # deriving 1 from PID / PID, to -1 (negative of that)
    # then joining with the * ''
    (_ = $$/$$)..-_
  ] * ''

# Y Combinator hack again, which takes a single arg:
#   __ is a string
(@__ = -> __ {
  # $_ is the first character of the string (__[0])
  $_ = __[_-_]

  # Check if the string still contains the character
  @_[$_] ? 
    # $* is ARGV, which is an empty array at this point
    # 
    # so we're concatting the position of the last character onto the array
    $* << (@_ =~ /#$_/) :
    (
      # So we're getting an array back and splatting it which is composed of
      # the last two characters of the array $*
      # 
      # If there aren't two characters we have
      #   ___ is nil
      #   $-- is nil
      ___, $__ = $*[-_-_, _+_];

      # Noted this breaks if you linebreak here, but for the sake of explaining...
      # 
      # If the first character is a plus, 'pop' the last two characters and replace them with:
      $*[-_-_,_+_] = $_[?+] ?
        # the result of their addition
        ___ + $__ :
        # or the result of their subtraction
        $_[?-] ?
          ___ - $__ :
          # or multiplication
          $_[?*] ?
            ___ * $__ :
            # or division
            $_[?/] ? ___ / $__ :
            # or just 1
            _
    )

    # Recurse if the accumulator isn't out of bounds
    __[_] ?
      # if not out of bounds, recall with strings tail
      @__[__[_..-_]] :
      # else return the accumulator
      $*
})