casevariants.nim

import eminim, std/[parsejson, streams]

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

proc initFromJson(dst: var Bar, p: var JsonParser) =
  var
    onceShared, onceKind, onceBad, onceBanana, onceApple = false
  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)
      if not onceShared:
        onceShared = true
        initFromJson(dst.shared, p)
      else:
        when defined(emiDuplicateKey): skipJson(p)
        else: raiseParseErr(p, "duplicate object field")
    of "kind":
      discard getTok(p)
      eat(p, tkColon)
      var kindTmp: Fruit
      if not onceKind:
        onceKind = true
        initFromJson(kindTmp, p)
      else:
        when defined(emiDuplicateKey): skipJson(p)
        else: raiseParseErr(p, "duplicate object field")
      if dst.kind != kindTmp:
        case dst.kind
        of Banana:
          if onceBad or onceBanana:
            when not defined(emiLenient): raiseParseErr(p, "valid object field")
          if onceShared:
            var tmpShared: int
            if onceShared: tmpShared = dst.shared
            dst = (typeof dst)(kind: kindTmp, shared: tmpShared)
        of Apple:
          if onceApple:
            when not defined(emiLenient): raiseParseErr(p, "valid object field")
          var tmpShared: int
          if onceShared: tmpShared = dst.shared
          dst = (typeof dst)(kind: kindTmp, shared: tmpShared)
    of "bad":
      if dst.kind != Banana:
        if onceApple:
          when not defined(emiLenient): raiseParseErr(p, "valid object field")
        var tmpShared: int
        if onceShared: tmpShared = dst.shared
        dst = (typeof dst)(kind: Banana, shared: tmpShared)
      discard getTok(p)
      eat(p, tkColon)
      if not onceBad:
        onceBad = true
        initFromJson(dst.bad, p)
      else:
        when defined(emiDuplicateKey): skipJson(p)
        else: raiseParseErr(p, "duplicate object field")
    of "banana":
      if dst.kind != Banana:
        if onceApple:
          when not defined(emiLenient): raiseParseErr(p, "valid object field")
        var tmpShared: int
        if onceShared: tmpShared = dst.shared
        dst = (typeof dst)(kind: Banana, shared: tmpShared)
      discard getTok(p)
      eat(p, tkColon)
      if not onceBanana:
        onceBanana = true
        initFromJson(dst.banana, p)
      else:
        when defined(emiDuplicateKey): skipJson(p)
        else: raiseParseErr(p, "duplicate object field")
    of "apple":
      if dst.kind != Apple:
        if onceBad or onceBanana:
          when not defined(emiLenient): raiseParseErr(p, "valid object field")
        var tmpShared: int
        if onceShared: tmpShared = dst.shared
        dst = (typeof dst)(kind: Apple, shared: tmpShared)
      discard getTok(p)
      eat(p, tkColon)
      if not onceApple:
        onceApple = true
        initFromJson(dst.apple, p)
      else:
        when defined(emiDuplicateKey): skipJson(p)
        else: raiseParseErr(p, "duplicate object field")
    else:
      when defined(emiLenient): skipJson(p)
      else: raiseParseErr(p, "valid object field")
    if p.tok != tkComma:
      break
    discard getTok(p)
  eat(p, tkCurlyRi)

let s = newStringStream("""[{"shared":1,"kind":"Apple","apple":"world"},{"bad":1.0,"kind":"Banana","shared":2}]""")
echo s.jsonTo(seq[Bar])