Mirror of forum.nim-lang.org

10536 :: Why can't you echo a ref object?

• moose (orginal) [2023-10-08T01:51:48+02:00] view original

  Bit of a noob question but can anyone help me understand why you cannot echo x in the below code but you can echo y. The error message says it expects x to be of type varargs[typed, `$]` and I can then fix it by writing my own toString proc like proc `$`(x:WithRef): string = x.name but I'd just like to know why adding the ref makes this happen.`

  type
    WithRef = ref object
      name: string
    
    NoRef = object
       name: string
  
  let
    x = WithRef(name: "wren")
    y = NoRef(name: "newt")
  
  echo x
  echo y
  Run

  Thank you!

---

• ElegantBeef (orginal) [2023-10-08T05:15:37+02:00] view original

  Nim's default $ are implemented for value types not reference types. You could implement:

  proc `$`(r: ref): string =
    if r.isNil:
      "nil"
    else:
      $r
  Run

  But this naive approach and will explode when using cyclical types.

---

• choltreppe (orginal) [2023-10-08T08:28:34+02:00] view original

  you forgot to deref r

  proc `$`(r: ref): string =
    if r.isNil:
      "nil"
    else:
      $r[]
  Run

---

• ElegantBeef (orginal) [2023-10-08T09:21:38+02:00] view original

  We'll pretend instead of failing to proofread it was an exercise for the dear reader.

---

• dlesnoff (orginal) [2023-10-08T22:48:22+02:00] view original

  This doesn't answer OP's question: > I'd just like to know why adding the ref makes this happen.`

  Is it also left as an exercise to the reader?

---

• ElegantBeef (orginal) [2023-10-08T23:11:44+02:00] view original

  I did answer the question, there is no $ defined for ref. The 'exercise to the reader' was a joke response due to my ineptitude to proofread a $ implementation. As I mentioned the issue is that a naive $ on a cyclical data type will enter an endless loop. So yes it's to avoid printing cyclical objects.

---