Mirror of forum.nim-lang.org

7530 :: Conjunctions of sum type parameters

• benob (orginal) [2021-02-18T19:05:05+01:00] view original

  I am playing with sum type parameters, and I'd like to specify types for parameters that play well together. For instance, I have a function that can take types (A, C) or (B, D) but I don't wont (A, D) or (B, C) to be valid. With sum types, f(A|B, C|D) makes every combo valid. Is there a way to impose that restriction?

  type A = object
  type B = object
  type C = object
  type D = object
  
  proc `$`(x: A): string = "A"
  proc `$`(x: B): string = "B"
  proc `$`(x: C): string = "C"
  proc `$`(x: D): string = "D"
  
  # f is overspecified
  proc f(x: A | B, y: C | D) =
    echo (x, y)
  
  # g is well specified
  proc g*(x: A, y: C) = f(x, y)
  proc g*(x: B, y: D) = f(x, y)
  
  f(A(), D())
  g(A(), C())
  g(B(), D())
  Run

---

• mratsim (orginal) [2021-02-18T19:17:02+01:00] view original

  You can use not A if you want, but in your case better use an inner internal proc (can be inline) with the implementation and 2 public overload for the public use and restriction. It would be much easier to read and understand.

---

• shirleyquirk (orginal) [2021-02-18T19:35:42+01:00] view original

  i see nothing wrong with your well-specified version, but there're many ways to achieve what you're after, including:

  proc ghelper(xy:(A,C)|(B,D)): echo xy
  
  proc g*[T,U](x:T,y:U)=ghelper((x,y))
  #or
  template g*(x,y) = ghelper((x,y))
  
  #or full manual:
  import macros
  proc g[T,U](x:T, y:U) =
    when (T is A and U is C) or (T is B and U is D):
      echo (x,y)
    else:
      static:error("A,C or B,D expected, got: " & $T & "," & $U)
  Run

---

• benob (orginal) [2021-02-18T20:39:25+01:00] view original

  Thanks, that's helpful.

---