Nim/tests/niminaction/Chapter3/ChatApp/src/protocol.nim

import json

type
  Message* = object
    username*: string
    message*: string

  MessageParsingError* = object of Exception

proc parseMessage*(data: string): Message {.raises: [MessageParsingError, KeyError].} =
  var dataJson: JsonNode
  try:
    dataJson = parseJson(data)
  except JsonParsingError:
    raise newException(MessageParsingError, "Invalid JSON: " &
                       getCurrentExceptionMsg())
  except:
    raise newException(MessageParsingError, "Unknown error: " &
                       getCurrentExceptionMsg())

  if not dataJson.hasKey("username"):
    raise newException(MessageParsingError, "Username field missing")

  result.username = dataJson["username"].getStr()
  if result.username.len == 0:
    raise newException(MessageParsingError, "Username field is empty")

  if not dataJson.hasKey("message"):
    raise newException(MessageParsingError, "Message field missing")
  result.message = dataJson["message"].getStr()
  if result.message.len == 0:
    raise newException(MessageParsingError, "Message field is empty")

proc createMessage*(username, message: string): string =
  result = $(%{
    "username": %username,
    "message": %message
  }) & "\c\l"

when true:
  block:
    let data = """{"username": "dom", "message": "hello"}"""
    let parsed = parseMessage(data)
    doAssert parsed.message == "hello"
    doAssert parsed.username == "dom"

  # Test failure
  block:
    try:
      let parsed = parseMessage("asdasd")
    except MessageParsingError:
      doAssert true
    except:
      doAssert false