Mirror of forum.nim-lang.org

8913 :: How to get the type of an expression that's a NimNode?

• RainbowAsteroids (orginal) [2022-02-14T07:19:56+01:00] view original

  I want to make a macro that would do the following:

  macro datatype(name, body: untyped) =
    discard # ...
  
  datatype Person:
    name: string
    age: Natural
    friends = newSeq[Person]()
  
  #[
  # Turns into
  type Person = object
    name: string
    age: Natural
    friends: seq[Person]
  
  proc initPerson(name: string, age: Natural, friends = newSeq[Person]()): Person =
    Person(name: name, age: age, friends: friends)
  ]#
  Run

  However, I'm confused about how to get the type of a given expression that's a NimNode. How would I go about doing that?

---

• DeletedUser (orginal) [2022-02-14T07:39:17+01:00] view original

  You can't. In this case, your best bet is probably to emit typeof(newSeq[Person]()). But the macro itself cannot be aware of the type without using a typed argument.

---

• Vindaar (orginal) [2022-02-14T10:44:09+01:00] view original

  As @Hlaaftana says using a typeof approach is one option.

  Another, but more complicated one, is to turn the macro logic into a 2 step process. Take the untyped macro and do the stuff you need the untyped tree for, then emit the code that needs type information to a 2nd - now typed - macro.

---

• mratsim (orginal) [2022-02-14T11:04:05+01:00] view original

  Nim gives you a lot of power but unfortunately that issue has become a personal pet peeve of mine. They are workarounds, what Vindaar mentioned, but oh boy you need to learn dancing on a tight rope:

  • https://github.com/nim-lang/RFCs/issues/44
  • https://github.com/nim-lang/Nim/issues/6785

---

• demotomohiro (orginal) [2022-02-14T11:26:53+01:00] view original

  I wrote a similar macro that create a object type definition from a constructor proc https://github.com/demotomohiro/objectDef/blob/master/src/objectDef.nim

  It works in the same way @Vindaar explained. objectDef macro takes a procedure with untyped parameter and create a AST that calls tupleCtorToObjectDef macro. tupleCtorToObjectDef takes generated AST as typed parameter and it can get the type with getTypeInst proc in macros module.

---