Mirror of forum.nim-lang.org

10554 :: Type introspection

• dlesnoff (orginal) [2023-10-18T18:00:31+02:00] view original

  I am going through examples of basic metaprogramming challenges, as proposed to me by the ChatGPT4 bot: https://chat.openai.com/share/9056f7cb-e20f-41ec-8a2f-c6b8ea0ba484

  One of these is type introspection: Given a Nim object or type, explore and print its fields using Nim's metaprogramming capabilities.

  Let's say I have an object defined with:

  type
    Person = object
      name: string
      age: int
  Run

  Can I list the attributes and the types associated to each attributes given:

  1. the definition of the type
  2. a variable of type Person
  3. the typedesc Person

  If so, how?

  Notice that answer to 1) is yes and is pretty easy, but I am not sure how to do it in the most generic way (supporting inheritance, pragmas, generics, object variants and so on …). How could I use getType, getImpl or getTypeImpl ?

  I know my question is a bit vague and theoretical. I do not have a project with a purpose in mind, I just want a more in depth tutorial to metaprogramming than the one given on the website. So I can't answer questions of the type: "Do you need to list attributes and their types at compile-time or runtime". My answer would probably be both.

  I don't mind

---

• ElegantBeef (orginal) [2023-10-18T19:29:05+02:00] view original

  Not that it's well documented, but a ton of type operations are defined inside https://github.com/beef331/micros/blob/master/src/micros/definitions/objectdefs.nim . Most important to this question would be https://github.com/beef331/micros/blob/master/src/micros/definitions/objectdefs.nim#L229-L241

  Another way of doing type introspection is just using the fieldPairs/fields iterator, you can use this in a variety of ways, the simplest is as follows.

  proc listStuffs(obj: object) =
    for name, field in obj.fieldPairs:
      echo name, ": ", field
  
  proc listStuffs(t: typedesc[object]) = listStuffs(default(t))
  Run

---