Dart Constant Constructors

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One of the many aspects of Dart that I do not cover in Dart for Hipsters is constant constructors. I briefly played with them a while back, but never really understood them. I will give them another try tonight.

A "regular" Dart cookie class (the yummy kind, not the browser obnoxious kind) might look and get used something like the following:

class Cookie {
  var number_of_chips;
  Cookie({this.number_of_chips});
}

main() {
  var cookie = new Cookie(number_of_chips: 12);

  print("An ordinary cookie has ${cookie.number_of_chips} chips");
}

Running this code would result in the following output:

An ordinary cookie has 12 chips

Constant constructors, as the name implies, involve instance variables that cannot be changed. In Dart, this is generally what the final keyword does—it creates variables (instance or otherwise) that cannot be updated.

Bearing that in mind, I create a PerfectCookie class, which is almost identical to my "regular" Cookie class, except that the number of chips is declared as a final value of 42:

class PerfectCookie {
  final number_of_chips = 42;
  const PerfectCookie();
}

Updating the main() entry point to use a PerfectCookie as well, I have:

main() {
  var cookie = new Cookie(number_of_chips: 12);
  var perfect_cookie = new PerfectCookie();

  print("An ordinary cookie has ${cookie.number_of_chips} chips");
  print("The perfect cookie has ${perfect_cookie.number_of_chips} chips");
}

The resulting output, as expected, now claims that 42 is the ideal number of chips in a cookie:

An ordinary cookie has 12 chips
The perfect cookie has 42 chips

Running this code through dart_analyzer generates no comments, so this seems a legitimate use-case for constant constructors.

Based on the description in the spec, I can also declare number_of_chips as final, but without assigning an actual value:

class PerfectCookie {
  final number_of_chips;
  const PerfectCookie({this.number_of_chips});
}

I can then use this class identically to the "regular" Cookie class:

main() {
  var cookie = new Cookie(number_of_chips: 12);
  var perfect_cookie = new PerfectCookie(number_of_chips: 42);

  print("An ordinary cookie has ${cookie.number_of_chips} chips");
  print("The perfect cookie has ${perfect_cookie.number_of_chips} chips");
}

I still get the same output:

An ordinary cookie has 12 chips
The perfect cookie has 42 chips

Most interestingly, the code still passes dart_analyzer. I had really expected dart_analyzer to complain that I was assigning a non-final value to number_of_chips in my constant constructor, but this seems fine.

What does not work (and is prohibited per the spec) is updating the number of chips in the object generated by the constant constructor:

main() {
  var cookie = new Cookie(number_of_chips: 12);
  var perfect_cookie = new PerfectCookie(number_of_chips: 42);

  cookie.number_of_chips = 13;
  perfect_cookie.number_of_chips = 43;

  print("An ordinary cookie has ${cookie.number_of_chips} chips");
  print("The perfect cookie has ${perfect_cookie.number_of_chips} chips");
}

I do not need dart_analyzer for this code. It produced a compile-time error:

Unhandled exception:
NoSuchMethodError : method not found: 'number_of_chips='
Receiver: Instance of 'PerfectCookie'
Arguments: [43]
#0      Object._noSuchMethod (dart:core-patch:1354:3)
#1      Object.noSuchMethod (dart:core-patch:1355:25)
#2      main (file:///Code/classes/constant_class.dart:16:18)

In this fashion, constant constructors seem a cheap way of creating objects with getters and no corresponding setter. It feels a little strange declaring this in the constructor rather than at the class name level. Is it possible to define a class with a constant constructor and non-constant constructor? And if so, to what end? Those minor worries aside, I can definitely see using these as some point—even if not often.


Day #613