First class support for object variants

[planetis-m]

Requires #12 to be implemented.

type
  Bar = object
    shared: int
    case kind: Fruit
    of Banana:
      bad: float
      banana: int
    of Apple: apple: string

let s = newStringStream("""{"kind":"Apple","apple":"world"}""")
let a = s.jsonTo(Bar)

proc initFromJson(dst: var Bar, p: var JsonParser) =
  eat(p, tkCurlyLe)
  while p.tok != tkCurlyRi:
    if p.tok != tkString:
      raiseParseErr(p, "string literal as key")
    case p.a
    of "shared":
      discard getTok(p)
      eat(p, tkColon)
      initFromJson(dst.apple, p)
    of "kind":
      discard getTok(p)
      eat(p, tkColon)
      var kindTmp`gensym0: Fruit
      initFromJson(kindTmp`gensym0, p)
      if dst.kind != kindTmp`gensym0:
        case dst.kind
        of Banana:
          if onceBad or onceBanana:
            raiseParseErr(p, "valid object field")
          if onceShared:
            dst = (typeof dst)(kind: kindTmp`gensym0, shared: dst.shared)
        of Apple:
          if onceApple:
            raiseParseErr(p, "valid object field")
          if onceShared:
            dst = (typeof dst)(kind: kindTmp`gensym0, shared: dst.shared)
        else: discard
    of "bad":
      if dst.kind != Banana: # not shared
        raiseParseErr(p, "valid object field")
      discard getTok(p)
      eat(p, tkColon)
      initFromJson(dst.bad, p)
    of "banana":
      if dst.kind != Banana:
        if onceApple:
          raiseParseErr(p, "valid object field")
        if onceShared:
          dst = (typeof dst)(kind: Banana, shared: dst.shared)
      discard getTok(p)
      eat(p, tkColon)
      initFromJson(dst.banana, p)
    of "apple":
      if dst.kind != Apple:
        if onceBad or onceBanana:
          raiseParseErr(p, "valid object field")
        if onceShared:
          dst = (typeof dst)(kind: Apple, shared: dst.shared)
      discard getTok(p)
      eat(p, tkColon)
      initFromJson(dst.apple, p)
    else:
      raiseParseErr(p, "valid object field")
    if p.tok != tkComma:
      break
    discard getTok(p)
  eat(p, tkCurlyRi)

[planetis-m]

Pretty bad, what if kind: "Apple" is not discovered yet. It would error on a valid json. Fixed.

[planetis-m]

Working POC: https://gist.github.com/planetis-m/00eb3ca829397e47417a4eb2f8036d60