control-flow

问题 Why does SwiftUI like to throw errors like Closure containing control flow statement cannot be used with function builder 'ViewBuilder' When I simply try something like VStack { for i in 0...10 { Text("Hello, world!") } } It will not compile. Why does swift care? How does the SwiftUI framework even detect if there are control flow statements, and throw errors? 回答1: How does the SwiftUI framework even detect if there are control flow statements, and throw errors? It doesn't. The Swift language

问题 Why does SwiftUI like to throw errors like Closure containing control flow statement cannot be used with function builder 'ViewBuilder' When I simply try something like VStack { for i in 0...10 { Text("Hello, world!") } } It will not compile. Why does swift care? How does the SwiftUI framework even detect if there are control flow statements, and throw errors? 回答1: How does the SwiftUI framework even detect if there are control flow statements, and throw errors? It doesn't. The Swift language

问题 I have a method that, depending on a predicate, will return one future or another. In other words, an if-else expression that returns a future: extern crate futures; // 0.1.23 use futures::{future, Future}; fn f() -> impl Future<Item = usize, Error = ()> { if 1 > 0 { future::ok(2).map(|x| x) } else { future::ok(10).and_then(|x| future::ok(x + 2)) } } This doesn't compile: error[E0308]: if and else have incompatible types --> src/lib.rs:6:5 | 6 | / if 1 > 0 { 7 | | future::ok(2).map(|x| x) 8 |

问题 I have a method that, depending on a predicate, will return one future or another. In other words, an if-else expression that returns a future: extern crate futures; // 0.1.23 use futures::{future, Future}; fn f() -> impl Future<Item = usize, Error = ()> { if 1 > 0 { future::ok(2).map(|x| x) } else { future::ok(10).and_then(|x| future::ok(x + 2)) } } This doesn't compile: error[E0308]: if and else have incompatible types --> src/lib.rs:6:5 | 6 | / if 1 > 0 { 7 | | future::ok(2).map(|x| x) 8 |

问题 I have a method that, depending on a predicate, will return one future or another. In other words, an if-else expression that returns a future: extern crate futures; // 0.1.23 use futures::{future, Future}; fn f() -> impl Future<Item = usize, Error = ()> { if 1 > 0 { future::ok(2).map(|x| x) } else { future::ok(10).and_then(|x| future::ok(x + 2)) } } This doesn't compile: error[E0308]: if and else have incompatible types --> src/lib.rs:6:5 | 6 | / if 1 > 0 { 7 | | future::ok(2).map(|x| x) 8 |

问题 I have a method that, depending on a predicate, will return one future or another. In other words, an if-else expression that returns a future: extern crate futures; // 0.1.23 use futures::{future, Future}; fn f() -> impl Future<Item = usize, Error = ()> { if 1 > 0 { future::ok(2).map(|x| x) } else { future::ok(10).and_then(|x| future::ok(x + 2)) } } This doesn't compile: error[E0308]: if and else have incompatible types --> src/lib.rs:6:5 | 6 | / if 1 > 0 { 7 | | future::ok(2).map(|x| x) 8 |

问题 I have a method that, depending on a predicate, will return one future or another. In other words, an if-else expression that returns a future: extern crate futures; // 0.1.23 use futures::{future, Future}; fn f() -> impl Future<Item = usize, Error = ()> { if 1 > 0 { future::ok(2).map(|x| x) } else { future::ok(10).and_then(|x| future::ok(x + 2)) } } This doesn't compile: error[E0308]: if and else have incompatible types --> src/lib.rs:6:5 | 6 | / if 1 > 0 { 7 | | future::ok(2).map(|x| x) 8 |

问题 I have a method that, depending on a predicate, will return one future or another. In other words, an if-else expression that returns a future: extern crate futures; // 0.1.23 use futures::{future, Future}; fn f() -> impl Future<Item = usize, Error = ()> { if 1 > 0 { future::ok(2).map(|x| x) } else { future::ok(10).and_then(|x| future::ok(x + 2)) } } This doesn't compile: error[E0308]: if and else have incompatible types --> src/lib.rs:6:5 | 6 | / if 1 > 0 { 7 | | future::ok(2).map(|x| x) 8 |

问题 I'm trying to make a connection to other columns within a function given a conditional. Essentially, I want to make a dataframe go from long to wide given a conditional, where those values in one column are NA relative to another column that has values in that same row, turn the NAs into a specific numeric. Although the values assigned have to be column specific. So if 2010 has NAs whilst 2019 has a value, then return 16 otherwise, if 2019 has NAs when in that same row, 2010 has values,

问题 I looked at the docs from Kotlin website, there are only two Control-Flow expressions: if and when . For if : the expression is required to have an else branch For when : The else branch is evaluated if none of the other branch conditions are satisfied. If when is used as an expression, the else branch is mandatory, unless the compiler can prove that all possible cases are covered with branch conditions. Question So it seems that there is no way to make a Control-Flow expression without