Equivalent of if let and guard let of Swift in Dart

Question

Just started Flutter with native iOS background, so I just have a quick question about Dart beta null safety.

So in Swift, because they have the idea of null safety from the beginning just like Kotlin, there are 2 features that I really like about the language is if let and guard let. These 2 make working with optional values so much easier. I'm not sure if the beta version of Dart has anything like that.

Thanks

Answer 1

I'm not an expert on Swift, but Dart will use null checks to automatically promote types, and I think that mostly does the job of if let and guard let.

For example:

String? x = possiblyReturnsNull();
if (x != null) {
  // All code within this block treats `x` as non-nullable.
}
// All code outside the block continues to treat `x` as nullable.

Note that promotion won't be performed on non-local variables, so for those you would need to explicitly introduce a local reference. (There is a language proposal to provide a mechanism to allow a nicer mechanism to add a local reference without polluting the outer scope.)

Answer 2

I'm going to jump in on this, since I'm coming from Swift too and love to use guard a lot. Adding to what @jamesdlin said, the opposite is also true.

So you can functionally do a Swift guard statement:

String? x = possiblyReturnsNull();
if (x == null) return whatever; // This works like Swift's guard
// All code outside the block now treats `x` as NON-nullable.

Answer 3

A small extension to the accepted answer is that Flutter also allows force-unwrapping an Optional. So in the case you are accessing a non-nil value that is not saved inside a variable, like in a dictionary, you need to unwrap it inside the if-statement:

if (someDict[someKey] != null) {
  print(someDict[someKey]!)
}
//