Evaluate to null if condition is met

[rubenferreira97]

Given Flutter uses dart as its programming language I will write this proposal here.
Flutter have a lot of optional arguments. Given that, most of the time there is a need to return/evaluate to null.

Some examples are:
Render an optional (custom) widget given a condition.

Center(
    child: renderWidget ? CustomWidget() : null,
)

Return no error message (null) if a condition is not met.

TextFormField(
    validator: (value) => nameController.text.isEmpty 
                          ? AppLocalizations.of(context).formEmptyValidation  
                          : null,
)

etc...

It would be reasonable to have a operator like ?* or something else in Dart?
nameController.text.isEmpty ?* AppLocalizations.of(context).formEmptyValidation

In fairness, some could argue that this seems an unnecessary operator in Dart, however in flutter development I would use this operator more than ??= for example.

[Levi-Lesches]

Personally, I use this pattern a lot too. My shortcut is to negate the condition so I can put null first, like so:

Center(
  child: !data.user.condition() ? null : ReallyBigWidget(
    children: [
      With(),
      Lots(),
      Of(),
      Parameters(),
    ]
  )
)

That way, I don't have to have the : null all the way at the end.

---

Also, I'd like an operator for only doing something if a value is non-null. This is different than ?. because it can be used in a for loop or passed to another function. For example:

class MyData {
  static List<MyData?> getList(List json) => [
    for (final dynamic nestedJson in json)
      nestedJson == null ? null : 
        MyData.fromJson(Map.from(nestedJson))
  ];
}

Using !! as an example operator, I'd love to rewrite this:

class MyData {
  static List<MyData?> getList(List json) => [
    for (final dynamic nestedJson in json)
      MyData.fromJson(Map.from(nestedJson!!))
  ];
}

Where !! would short-circuit the expression an evaluate to null (EDIT: See @ramsestom's better syntax below)

[ramsestom]

Using !! as an example operator, I'd love to rewrite this:

class MyData {
  static List<MyData?> getList(List json) => [
    for (final dynamic nestedJson in json)
      MyData.fromJson(Map.from(nestedJson!!))
  ];
}

Where !! would short-circuit the expression an evaluate to null

I agree that an operator that would allow to do someting if a value isn't null would be usefull.
Personaly I would be more in favor of an operator that would counter mimic the ?? operator. For example If this operator was named !! your example would write as:

class MyData {
  static List<MyData?> getList(List json) => [
    for (final dynamic nestedJson in json)
      nestedJson !! MyData.fromJson(Map.from(nestedJson))
  ];
}

[Levi-Lesches]

Yep, that's definitely a better way of putting it. I'd love to see it happen

[ykmnkmi]

Also

a !!= b;

like

if (b != null) a = b;

[ramsestom]

Also

a !!= b;

like

if (b != null) a = b;

well a !!= b would be equivalent to

if (a != null) a = b;

but yes, the idea is to have an operator that behave exactly as the oposite of ??
so you could do things like

a !!= tranformFunction(a); //a function that that would transform a if not null

if you want

[Levi-Lesches]

Regarding the initial proposal, how about modifying condition ? whenTrue : whenFalse to make the : whenFalse part optional? I don't know if this will conflict with other grammar rules, but it would make sense, given that "if the value is not there, set it to null" is pretty intuitive.

// These two are equivalent
final String? name = isSignedIn ? getUsername(Auth.user);
final String? name = isSignedIn ? getUsername(Auth.user) : null;

EDIT: Pretty confident it can be done since currently Dart throws an error saying expected ":", which means it must be pretty unambiguous that the ? is meant for ternary and not null-safety.

[Levi-Lesches]

I found a list of similar null operators over here, thanks to @lrhn. Specifically,

• !!: Proposal: operator !! (opposite of ??) #361
• ?*: Null safety: null-aware operator for constructor or function call arguments #1039 (defined as ?! or !? in that issue)
• ?param: Proposal: null-elimination operator - prefix "?" #219 (only provide parameter if not null)
• a !!= b: Operator =?? (opposite to ??=) #288 (defined as =??)