pattern-matching

问题 I'm currently looking into Rust macros and I can not find any detailed documentation on repetitions. I would like to create macros with optional parameters. This would be my idea: macro_rules! single_opt { ($mand_1, $mand_2, $($opt:expr)* ) =>{ match $opt { Some(x) => println!("1. {} 2. {}, 3. {}", $mand_1, $mand_2, x); None => single_opt!($mand_1, $mand_2, "Default"); } } } fn main() { single_opt!(4,4); } This example seems to be outdated, since I can not compile it. The Rust book mentions

问题 I'm currently looking into Rust macros and I can not find any detailed documentation on repetitions. I would like to create macros with optional parameters. This would be my idea: macro_rules! single_opt { ($mand_1, $mand_2, $($opt:expr)* ) =>{ match $opt { Some(x) => println!("1. {} 2. {}, 3. {}", $mand_1, $mand_2, x); None => single_opt!($mand_1, $mand_2, "Default"); } } } fn main() { single_opt!(4,4); } This example seems to be outdated, since I can not compile it. The Rust book mentions

问题 abstract class IntTree object Empty extends IntTree case class NonEmpty(elem: Int, left: IntTree, right: IntTree) extends IntTree def assertNonNegative[S <: IntTree](t: S): S = { t match { case Empty => Empty // type mismatch, required: S, found: Empty.type case NonEmpty(elem, left, right) => if (elem < 0) throw new Exception else NonEmpty(elem, assertNonNegatve(left), assertNonNegative(right)) // req: S, fd: NonEmpty.type } } This is my failed attempt of implementing the function with

问题 Closed. This question needs debugging details. It is not currently accepting answers. Want to improve this question? Update the question so it's on-topic for Stack Overflow. Closed 5 hours ago . I came across this amazing topic on https://www.baeldung.com/java-pattern-matching-instanceof. But when I try to run the following code, it throws compile time error: if(obj instanceof String s) { System.out.println(s); } I am using Java 14. 回答1: This is a preview feature in Java 14, see JEP 305 and

问题 Closed. This question needs debugging details. It is not currently accepting answers. Want to improve this question? Update the question so it's on-topic for Stack Overflow. Closed 5 hours ago . I came across this amazing topic on https://www.baeldung.com/java-pattern-matching-instanceof. But when I try to run the following code, it throws compile time error: if(obj instanceof String s) { System.out.println(s); } I am using Java 14. 回答1: This is a preview feature in Java 14, see JEP 305 and

问题 I have the following code: val byteBuffer = array(0) match { case _: Int => ByteBuffer.allocate(4 * array.length) case _: Long => ByteBuffer.allocate(8 * array.length) case _: Float => ByteBuffer.allocate(4 * array.length) case _: Double => ByteBuffer.allocate(8 * array.length) case _: Boolean => ByteBuffer.allocate(1 * array.length) } How can I convert it to use type class? Edit: I was asked what the type of the array is. It's complicated. The array is declared like that: val array = obj

问题 Assume I have follwing code def get[T](name:String)(implicit mf:ClassManifest[T]):T = mf.erasure match { case classOf[Boolean] => obj.getBoolean(name) case classOf[Int] => obj.getInt(name) } Now code dosn't work because classOf[Int] is invalid match value. 回答1: You should almost certainly investigate alternatives to using manifests and matching on class objects. In this case type classes will provide a much cleaner solution, // Assuming that Store is the type of obj ... trait Get[T] { def get

问题 I'm trying to use Scala pattern matching on Java Class[_] (in context of using Java reflection from Scala) but I'm getting some unexpected error. The following gives "unreachable code" on the line with case jLong def foo[T](paramType: Class[_]): Unit = { val jInteger = classOf[java.lang.Integer] val jLong = classOf[java.lang.Long] paramType match { case jInteger => println("int") case jLong => println("long") } } Any ideas why this is happening ? 回答1: The code works as expected if you change

问题 I'm trying to use Scala pattern matching on Java Class[_] (in context of using Java reflection from Scala) but I'm getting some unexpected error. The following gives "unreachable code" on the line with case jLong def foo[T](paramType: Class[_]): Unit = { val jInteger = classOf[java.lang.Integer] val jLong = classOf[java.lang.Long] paramType match { case jInteger => println("int") case jLong => println("long") } } Any ideas why this is happening ? 回答1: The code works as expected if you change

问题 Working on a Haskell project, I'm dealing with the Event data type from the FSNotify package. The constructors for Event are all: Added FilePath UTCTime Modified FilePath UTCTime Removed FilePath UTCTime In my code, I'm only interested in extracting the FilePath from the Event and doing the same action regardless of the type constructor; because of this, I'm tempted to make a lambda. Unfortunately, the code suffers reduced readability when I drop a case expression into the lambda to pattern