Introduce fix keyword as syntactic sugar for local, promotable, nullable field

[weenzeel]

I've been reading the comments about if-variables.

What if we introduced a "fix" keyword, so that we could do ...

class Coffee {
  String? temperature;

  void heat() { temperature = 'hot'; }
  void chill() { temperature = 'iced'; }

  void checkTemp() {
    
    fix temperature;
    
    if (temperature != null) {
       print('Ready to serve ' + temperature + '!');
    } else {
      temperature = 'UNKNOWN';
    }


    // If we want the unfixed value we can just refer to it via this.temperature
        
  }

  String serve() => temperature! + ' coffee';
}

The code above would be equal to ...

class Coffee {
  String? temperature;

  void heat() { temperature = 'hot'; }
  void chill() { temperature = 'iced'; }

  void checkTemp() {
    
    var fixedTemperature = temperature;
    
    if (fixedTemperature != null) {
       print('Ready to serve ' + fixedTemperature + '!');
    } else {
      fixedTemperature = 'UNKNOWN';
      temperature = 'UNKNOWN';
    }
  }

  String serve() => temperature! + ' coffee';
}

• Fixing a field is like creating a local copy at a fixed point in time. That copy persists until the end of the scope.
• Writing to a fixed field is like updating a local copy in sync with the original field.
• The unfixed field can always be fetched via this.field.
• A fixed field can be refixed to refresh the fixed value.

Just throwing it out there :)

[mateusfccp]

This seems to be very similar to #1514.

[weenzeel]

This seems to be very similar to #1514.

@mateusfccp You're right. It's basically the same idea, just a different keyword. Thanks for making the connection!

[mit-mit]

Closing as a duplicate of #1514