How is single equals possibly in conditions?

Question

I am wondering why booleans work with either single or double equals in Dart, while the same is not true for other types like String

this works:

void main() {
  bool a = true;
  bool b = true;
  if (a == b) {
    print('Hello, World!');
  }
}

This works, too:

void main() {
  bool a = true;
  bool b = true;
  if (a = b) {
    print('Hello, World!');
  }
}

but this fails (does not compile):

void main() {
  String a = 'true';
  String b = 'true';
  if (a = b) {
    print('Hello, World!');
  }
}

But my main motivation why I've started the post is that: the same logic (single equals) does not work in my code (double is OK, and finding this took my two days). What I mean is below code compiles but condition with single equal never satisfies unless i use double equals:

                            if (gxcG.findWealth(model.symbol).value.isstillowned =
                                    false)
                                  {
                                    
                                    gxcG.marketOBought.forEach((element) {
                                      if (element.value.symbol ==
                                          model.symbol) {
                                        element.value.isstillowned = false;
                                      }
                                    }),
                                    if (model.portfolioname != null)
                                      {
                                        gxcForRO
                                            .find(true, model.portfolioname!)
                                            .value
                                            .marketOBought = gxcG.marketOBought,
                                      },
                                    gxcG.wealth.removeWhere((element) =>
                                        element.value.symbol == model.symbol),
                                  },

gxcG is a GetXController extended class, and 'findWealth' method is as below:

 Rx<WealthModel> findWealth(String symbol) {
    return wealth.firstWhere((element) => element.value.symbol == symbol);
  }

Answer 1

What you are doing here:

  bool a = true;
  bool b = true;
  if (a = b){

Is setting a to b and then return the value assigned to a. Since this value is a boolean, this is "allowed" but should really not be something you are doing since it is horrible (also, you are changing a instead of just comparing).

You can see this change if you print a and b afterwards in an example like this:

void main() {
  bool a = true;
  bool b = false;

  if (a = b) {
    print('Hello, World!');
  }

  print(a); // false
  print(b); // false
}

The reason why this example fails:

void main() {
  String a = 'true';
  String b = 'true';
  if (a = b) {
    print('Hello, World!');
  }
}

Is because we set a to the value of b and return this value to the if-statement. But since b is a String, we cannot use this in our if-statement since it needs an expression resulting in a boolean.

Here is an (quick) example of how this syntax "can" be helpful:

void main() {
  String a;

  if ((a = getData()).isNotEmpty) {
    print('We got data: $a'); // We got data: Here is some data
  }
}

String getData() => 'Here is some data';

So since (a = getData()) will return the value we assign to a (so a String in this case) we can use that to ask if this value is empty or not.