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7414 :: Unmarshal JSON to type

• vinyll (orginal) [2021-01-22T10:02:29+01:00] view original

  Hey there,

  I'm a Python dev getting into Nim.

  As the paradigm is very new to me, I'm wondering how to convert a JSON string into a type.

  Here's what I have:

  
  import json
  
  type
    Asset = object
      percentage: float
  
  let content = """[{"percentage": "0.5"}, {"percentage": "0.9"}]""".parseJson()
  
  to(content, Asset[0])
  Run

  I get the following error:

  
  Incorrect JSON kind. Wanted '{JInt, JFloat}' in '.percentage' but got 'JString'
  Run

  I'm aware the JSON is a string and should be a float, but I can't modify it (remove server providing it).

  What would be the best practice to map the json to my Asset? Anyway to cast automagically or do I have to convert manually each value before unmarshalling into Asset?

  Thanks Nim community! :)

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• lqdev (orginal) [2021-01-22T10:21:57+01:00] view original

  You can try using jsonutils, which allows you to provide your own hooks for unmarshalling. Alternatively you can just make the field into a string and use parseFloat on it later.

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• ynfle (orginal) [2021-01-22T10:51:14+01:00] view original

  Here is a slight modification that works https://play.nim-lang.org/#ix=2MSu. There were 2 issues with the code.

  1. to(content, Assest[0]) doesn't work. You probably meant to write content[0]
  2. The numbers were quoted with " so it recognized them as strings. In the modified example I removed the quotations around the floats

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• mratsim (orginal) [2021-01-22T11:01:17+01:00] view original

  There are a lot of third-party libraries:

  • eminim: https://github.com/planetis-m/eminim
  • jason: https://github.com/disruptek/jason
  • jsony: https://github.com/treeform/jsony
  • nim-json-serialization: https://github.com/status-im/nim-json-serialization
  • packedjson: https://github.com/Araq/packedjson

  Deserialization usage can be seen in jsony repo: https://github.com/treeform/jsony/blob/35d9fa6/tests/bench.nim#L13-L57

  Why so many? Some kind of json frenzy took Nim developers in January. Origin of the pandemic unknown, exercise caution.

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• treeform (orginal) [2021-01-22T19:54:28+01:00] view original

  Using jsony you can define a parseHook() that optionally checks if the float you want is actually a string:

  import jsony, print, strutils
  
  proc parseHook(s: string, i: var int, v: var float) =
    ## Optionally parse floats strings as real floats.
    ## Example: "0.9" -> 0.9
    eatSpace(s, i)
    if s[i] == '"':
      var floatStr: string
      parseHook(s, i, floatStr)
      v = parseFloat(floatStr)
    else:
      v = parseFloat(parseSymbol(s, i))
  
  type
    Asset = object
      percentage: float
  
  let content = """[{"percentage": "0.5"}, {"percentage": "0.9"}]"""
  
  let assets = content.fromJson(seq[Asset])
  print assets
  Run

  But if you can change the json that is passed to you, i would just store them as floats to start with with.

  Also if you are dealing with a ton of json, jsony is pretty fast, faster than std lib json.

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• alexeypetrushin (orginal) [2021-01-22T21:29:51+01:00] view original

  Why so many?

  I'd say it could be a sign that STD JSON library is not good and needs improvement.

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• srbga (orginal) [2021-01-24T13:11:43+01:00] view original

  Yes, jsony is very fast. In our project I successfully cut down the runtime of an experimental nim plugin (by compiling nim to C++ and embed into the codebase) from 7+ ms to 1.2ms by replacing STD json to jsony. It's a huge improvement because that code is running inside a render loop. Thank you for the awesome library!

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