Generics trouble in Pony

parser.pony

interface ParseBuffer
  fun peek(): U8 ?
  fun ref head(): U8 ?
  fun eof(): Bool
  fun clone(): ParseBuffer


class _StringBuffer is ParseBuffer
  let _str: String val
  var _pos: USize
  new create(s: String val) =>
    _str = s
    _pos = 0
  new _positioned(s: String val, p: USize) =>
    _str = s
    _pos = p

  fun peek(): U8 ? =>
    _str(_pos)

  fun ref head(): U8 ? =>
    let c = _str(_pos)
    _pos = _pos + 1
    c

  fun eof(): Bool => _pos >= _str.size()
  fun clone(): ParseBuffer => _StringBuffer._positioned(_str, _pos)


interface Parser[A]
  fun op_or(p: Parser[A]): Parser[A] =>
    PTry[A](this, p)

  fun parse(buf: ParseBuffer): A ?

class PTry[A] is Parser[A]
  """Tries the left-hand parser. If it succeeds, returns that value. If it
  fails, tries the right-hand parser."""
  let _lhs: Parser[A] box
  let _rhs: Parser[A] box
  new create(l: Parser[A] box, r: Parser[A] box) =>
    _lhs = l
    _rhs = r

  fun parse(buf: ParseBuffer): A ? =>
    let copy = buf.clone()
    try
      _lhs.parse(buf)
    else
      _rhs.parse(copy)
    end

class PSeq[A] is Parser[Array[A]]
  """Performs each parser in sequence, returning an Array."""
  let _parsers: Array[Parser[A] box] box
  new create(parsers: Array[Parser[A] box] box) =>
    _parsers = parsers

  fun parse(buf: ParseBuffer): Array[A] ref ? =>
    var ret: Array[A] ref = Array[A]()
    for p in _parsers.values() do
      let res: A = p.parse(buf)
      ret.push(res)
    end
    ret