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1382 :: noob: json to object conversion

• pyloor (orginal) [2015-06-29T12:13:44+02:00] view original

  Hi,

  i am struggling with the json module. Currently i am trying to map a json from file to an object. Here is the code:

  import json
  
  type Config = object
      rootDev: string
      grubDevLabel: string
      title: string
      updateGrubDir: string
  
  proc readConfig(): Config =
      result = parseFile("grubify.json")
  
  when isMainModule:
      var config: Config = readConfig()
      echo(config.rootDev)
  Run

  the error message is

  parseJsonFile.nim(10, 23) Error: type mismatch: got (JsonNode) but expected 'Config'
  Run

  What is JsonNode. I read the documentation of the json module but I got not englighted enough to understand how to work with a jsonNode...

  Thanks for your help.

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• Phaal (orginal) [2015-06-29T12:58:16+02:00] view original

  Hi,

  JsonNode is return type of parseFile function.

  You must assign each field of JsonNode to its corresponding field in your Config object.

  Look at the example at the start of json documentation:

  http://nim-lang.org/docs/json.html

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• Araq (orginal) [2015-06-29T13:51:14+02:00] view original

  This can also be done via a macro. Somebody should write one.

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• pyloor (orginal) [2015-06-29T14:48:49+02:00] view original

  @Phaal i wasn't sure that i have to manually assign the json fields to the object. now its clear and it works now

  proc readConfig(): Config =
      var js: JsonNode
      js = parseFile("grubify.json")
      result.rootDev = js["rootDev"].str
      result.grubDevLabel = js["grubDevLabel"].str
      result.title = js["title"].str
      result.updateGrubDir = js["updateGrubDir"].str
  Run

  the documentation lacks on some edges with examples. But the git repos of Dom96 and def- are great. The most parts I have learned so far are from their repos. I read in the github issues tracker that there is an issue of adding more examples, but I have no idea whether my help is useful, because I have much more to learn how nim works.

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• andrea (orginal) [2015-06-29T15:23:47+02:00] view original

  By the way, there is no need to make js mutable, you could simply do

  proc readConfig(): Config =
    let js = parseFile("grubify.json")
    result.rootDev = js["rootDev"].str
    result.grubDevLabel = js["grubDevLabel"].str
    result.title = js["title"].str
    result.updateGrubDir = js["updateGrubDir"].str
  Run

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• andrea (orginal) [2015-06-30T17:24:08+02:00] view original

  Also, have a look at the marshal module

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• federico3 (orginal) [2016-11-25T16:20:17+01:00] view original

  I'm told that while marshal uses JSON, there is no guarantee that it will not change.

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• flyx (orginal) [2016-11-26T19:25:44+01:00] view original

  Since YAML is a superset of JSON, you can use NimYAML to deserialize the JSON file. It is able to automatically map a JSON object to a Nim object:

  import yaml.serialization, streams
  
  type Config = object
    rootDev: string
    grubDevLabel: string
    title: string
    updateGrubDir: string
  
  proc readConfig(): Config =
    var s = newStringStream("""
  {
    "rootDev": "rootDevValue",
    "grubDevLabel": "gdlValue",
    "title": "titleValue",
    "updateGrubDir": "ugdValue"
  }
    """)
    load(s, result)
  
  let config = readConfig()
  Run

  To load a file, simply use a FileStream instead of a StringStream. Note that the names of the object's fields must match exactly.

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