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10929 :: Definition of name equivalence

• sls1005 (orginal) [2024-01-30T10:03:59+01:00] view original

  According to https://nim-lang.org/docs/manual.html#type-relations-type-equality, name equivalence is used for objects, enumerations, distinct and generic types.

  If name equivalence here means that two types are equal if and only if they have the same name, we can infer that

  type A = object
  type B = object
  echo A is B #false
  echo B is A #false
  Run

  However, it cannot explain

  type C = object
  type D = C
  #`C` and `D` also have different names.
  echo C is D #true
  echo D is C #true
  Run

  I've searched in the page but couldn't find further explanations about name equivalence.

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• ElegantBeef (orginal) [2024-01-30T10:06:26+01:00] view original

  type D = C is a type alias as such C is D and ` D is C` , whilst a new type is introduced the type is just a new name for the right hand side.

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• xigoi (orginal) [2024-01-30T10:11:23+01:00] view original

  The term “name equivalence” is a bit misleading. It really means that each object/enum/distinct declaration creates a different type, even if the declaration looks the same.

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• penguinite (orginal) [2024-01-30T19:41:43+01:00] view original

  Yeah, as xigoi said, if you just add the distinct label to D then it will return false.

  Here is a code example:

  type
    C = object
    D = distinct C
  
  echo C is D # false
  Run

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• sls1005 (orginal) [2024-01-31T05:58:56+01:00] view original

  Then, I think it can be expressed with following rules:

  Type T is equal to type S if:

  1. None of T and S is object, enumeration, concept, pointer-to-object, reference-to-object, or a distinct or generic type, while T is structurally equivalent to S.
  2. T is object, enumeration, pointer-to-object, reference-to-object, or a distinct or generic type and is declared in the form, or any equivalent form of, type T = S, where S is not the keyword object, the keyword enum, the keyword distinct, the phrase ptr object, or the phrase ref object.
  3. T is object, enumeration, pointer-to-object, reference-to-object, or a distinct or generic type, and is declared in the form, or any equivalent form of, type T = U, where U, by rule 2, is a type equal to S.

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• Araq (orginal) [2024-01-31T08:39:30+01:00] view original

  Yes or like so:

  Types T is equal to type S if T and S are both nominal types and refer to the same declaration or if both are structural types and their structures are equal.

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