4 ms·

Mutating and non-mutating Swift contexts

36 points by tempodox 5y ago

bartvk 5y ago
What I find crazy in this context, is the following: struct ToggleExample: View { @State private var isWarningEnabled = false var body: some View { VStack { Text("Hello, world!") .foregroundColor(self.isWarningEnabled ? .red : .primary) Button(action: { self.isWarningEnabled.toggle() }, label: { Text("Hit me to toggle") }) } } } As you notice, the button action is a closure. In this case, a single line. Thus, I would expect that the following shorter code would work: Button(action: self.isWarningEnabled.toggle) { Text("Hit me to toggle") } That fails however with the following error message: Partial application of 'mutating' method is not allowed In a sense, this is logical. The `toggle()` is `mutating` and if we'd be able to save it as a closure, then we could run it from anywhere. That's like a reference value, when we're actually talking about a struct. But with SwiftUI, we're already in this gray area.
- tempodox 5y ago
  I think you misunderstood. The `action` closure is invoked with no arguments, but the invocation `self.isWarningEnabled.toggle()` does pass one argument, namely the `self` value from the closure's lexical context (`.toggle()` needs to know which instance to manipulate). If you replace the no-argument closure with a function that expects one argument (`self.isWarningEnabled.toggle`), you get the partial application the error mentions (“partial application” is passing fewer arguments to a function than it expects).
  - bartvk 5y ago
    Aha, of course... much appreciated, the penny dropped!
- willyt 5y ago
  I don’t write swift, but didn’t you miss a comma and a label for your Text(“Hit me to toggle”) argument in your second example? Did you mean: Button(action: self.isWarningEnabled.toggle, label:{ Text("Hit me to toggle") }) Not sure if you can just pass the method signature and omit the parentheses so it doesn’t get called like you can in python. Would be interested to know.
  - deergomoo 5y ago
    > I don’t write swift, but didn’t you miss a comma and a label Swift has a feature called "trailing closures". If the last argument to a function is a closure, you can close the brackets early and put the closure directly after.
andrekandre 5y ago
Swift’s concept of structured mutations only applies to value types, not to reference types like classes and actors. i feel like this would be even more usefull for classes.... it would great if classes could be read-only by default and you had to specify clearly what operations was able to mutate it... but perhaps that would make working with classes more cumbersome...
Shadonototro 5y ago
Swift is such a nice language to work with, and SwiftUI puts to shame everything both Windows/Android has to offer, it's not even close.. minus xcode of course...
- myko 5y ago
  I mostly agree re: the language itself but Compose is a lot more production ready than SwiftUI, unless you're only targeting iOS 14+, or really 15+ SwiftUI has horrendous bugs in each . release of iOS 13 and 14 has plenty of them too. It really needs to be deployed a la Jetpack Compose on Android, being tied to the OS makes it very difficult to ship reliable code using it.

36 points by tempodox 5y ago

What I find crazy in this context, is the following: struct ToggleExample: View { @State private var isWarningEnabled = false var body: some View { VStack { Text("Hello, world!") .foregroundColor(self.isWarningEnabled ? .red : .primary) Button(action: { self.isWarningEnabled.toggle() }, label: { Text("Hit me to toggle") }) } } } As you notice, the button action is a closure. In this case, a single line. Thus, I would expect that the following shorter code would work: Button(action: self.isWarningEnabled.toggle) { Text("Hit me to toggle") } That fails however with the following error message: Partial application of 'mutating' method is not allowed In a sense, this is logical. The `toggle()` is `mutating` and if we'd be able to save it as a closure, then we could run it from anywhere. That's like a reference value, when we're actually talking about a struct. But with SwiftUI, we're already in this gray area.

tempodox 5y ago
I think you misunderstood. The `action` closure is invoked with no arguments, but the invocation `self.isWarningEnabled.toggle()` does pass one argument, namely the `self` value from the closure's lexical context (`.toggle()` needs to know which instance to manipulate). If you replace the no-argument closure with a function that expects one argument (`self.isWarningEnabled.toggle`), you get the partial application the error mentions (“partial application” is passing fewer arguments to a function than it expects).
- bartvk 5y ago
  Aha, of course... much appreciated, the penny dropped!
willyt 5y ago
I don’t write swift, but didn’t you miss a comma and a label for your Text(“Hit me to toggle”) argument in your second example? Did you mean: Button(action: self.isWarningEnabled.toggle, label:{ Text("Hit me to toggle") }) Not sure if you can just pass the method signature and omit the parentheses so it doesn’t get called like you can in python. Would be interested to know.
- deergomoo 5y ago
  > I don’t write swift, but didn’t you miss a comma and a label Swift has a feature called "trailing closures". If the last argument to a function is a closure, you can close the brackets early and put the closure directly after.

andrekandre 5y ago

Swift’s concept of structured mutations only applies to value types, not to reference types like classes and actors. i feel like this would be even more usefull for classes.... it would great if classes could be read-only by default and you had to specify clearly what operations was able to mutate it... but perhaps that would make working with classes more cumbersome...

Shadonototro 5y ago

Swift is such a nice language to work with, and SwiftUI puts to shame everything both Windows/Android has to offer, it's not even close.. minus xcode of course...

myko 5y ago
I mostly agree re: the language itself but Compose is a lot more production ready than SwiftUI, unless you're only targeting iOS 14+, or really 15+ SwiftUI has horrendous bugs in each . release of iOS 13 and 14 has plenty of them too. It really needs to be deployed a la Jetpack Compose on Android, being tied to the OS makes it very difficult to ship reliable code using it.