Re: Self-referential tuples?

[Brian Rogoff via Digitalmars-d]

On Tuesday, 9 June 2015 at 23:48:05 UTC, Timon Gehr wrote:
'This' is a cute hack, but it doesn't replace a proper template 
fixpoint operator.

Could you explain this a bit more, maybe using some pseudo-D for 
your proposed template fixpoint operator? Thanks.

I don't think I'd need such a thing for the JSON AST example, 
since it isn't parameterized by type. There, the D mapping looks 
clumsy because, as Andrei noticed, the 'fields' are not named. If 
you map the OCaml straight to D you get something like what I 
write below, and you want both the Kind and the internal 
structure to be non-private so you can write functions which work 
on them. It's the same as tagged unions/variants in other 
languages but so much easier to use.

enum Kind {
  Bool,
  Number,
  String,
  Null,
  Array,
  Object
}

alias JSON = JSONStruct *;

struct JSONStruct {
public:
  Kind kind;
  union {
    bool jsonBool;
    double jsonNumber;
    string jsonString;
    JSON[] jsonArray;
    JSON[string] jsonObject;
  }

  this(Kind kind, bool jsonBool)
  in { assert(kind == Kind.Bool); }
  body {
    kind = Kind.Bool;
    this.jsonBool = jsonBool;
  }

  this(Kind kind, double jsonNumber)
  in { assert(kind == Kind.Number); }
  body {
    kind = Kind.Number;
    this.jsonNumber = jsonNumber;
  }

  this(Kind kind, string jsonString)
  in { assert(kind == Kind.String); }
  body {
    kind = Kind.String;
    this.jsonString = jsonString;
  }

  this(Kind kind)
  in { assert(kind == Kind.Null); }
  body {
    kind = Kind.Null;
  }

  this(Kind kind, JSON[] jsonArray)
  in { assert(kind == Kind.Array); }
  body {
    kind = Kind.Array;
    this.jsonArray = jsonArray;
  }

  this(Kind kind, JSON[string] jsonObject)
  in { assert(kind == Kind.Object); }
  body {
    kind = Kind.Object;
    this.jsonObject = jsonObject;
  }
}