pattern-matching

问题 Consider: sealed trait A case object B extends A case object C extends A case object D extends A sealed trait Test { type AA <: A val aa: AA } object Test { type Aux[AAA <: A] = Test { type AA = AAA } } def compilesOk(t: Test) = t.aa match { case _: B.type => println("B") case _: C.type => println("C") case _: D.type => println("D") } def compileError(t: Test) = t.aa match { case B => println("B") case C => println("C") case D => println("D") } The compileError function fails to compile with

问题 I have a question on pattern matching: Is it possible to somehow match a (string ++ [char] ++ anotherstring)? I have tried something like: f (s++";"++r) = s++r (the rhs is trivial, but its just for testing ;)) But this results in a parse error. 回答1: No, it's not possible. Pattern matching deconstructs values according to the constructors they were built with, so you can only use constructor applications in pattern matching to describe which values match the pattern and which don't. For

问题 I am doing my graduation project in the field of computer vision, and i have only taken one course in statistics that discussed very basic concepts, and now i am facing more difficulty in rather advanced topics, so i need help (book, tutorial, course, ..etc) to grasp and review the basic ideas and concepts in statistics and then dive into the details ( statistical details ) used in computer vision. 回答1: I assume you want something around the pattern recognition and machine learning fields. If

问题 I am doing my graduation project in the field of computer vision, and i have only taken one course in statistics that discussed very basic concepts, and now i am facing more difficulty in rather advanced topics, so i need help (book, tutorial, course, ..etc) to grasp and review the basic ideas and concepts in statistics and then dive into the details ( statistical details ) used in computer vision. 回答1: I assume you want something around the pattern recognition and machine learning fields. If

问题 I am doing my graduation project in the field of computer vision, and i have only taken one course in statistics that discussed very basic concepts, and now i am facing more difficulty in rather advanced topics, so i need help (book, tutorial, course, ..etc) to grasp and review the basic ideas and concepts in statistics and then dive into the details ( statistical details ) used in computer vision. 回答1: I assume you want something around the pattern recognition and machine learning fields. If

问题 I'm a little confused regarding pattern matching on a list in Scala. For example. val simplelist: List[Char] = List('a', 'b', 'c', 'd') //> simplelist : List[Char] = List(a, b, c, d) def simple_fun(list: List[Char]) = list match { case (x:Char) :: (y:List[Char]) => println(x) case _ => Nil } //> simple_fun: (list: List[Char])Any simple_fun(simplelist) //> a //| res0: Any = () This currently prints only one line of output. Should it not run/pattern match on each element of the List ? EDIT: I

问题 I'm a little confused regarding pattern matching on a list in Scala. For example. val simplelist: List[Char] = List('a', 'b', 'c', 'd') //> simplelist : List[Char] = List(a, b, c, d) def simple_fun(list: List[Char]) = list match { case (x:Char) :: (y:List[Char]) => println(x) case _ => Nil } //> simple_fun: (list: List[Char])Any simple_fun(simplelist) //> a //| res0: Any = () This currently prints only one line of output. Should it not run/pattern match on each element of the List ? EDIT: I

问题 I'm trying to match **String Newline String** pattern in a function Split. split::String -> [String] split[] = [] split (x++'\n':xs) = [x]++split(xs) I'm getting this error: Parse error in pattern: x ++ ('\n' : xs) What am I doing wrong here? I know there are other ways of achieving the same result but I'd like to understand what wrong with this pattern. I'm fairly new to Haskell BTW. 回答1: One problem (as I understand it) is that ++ is not a constructor of the list data type the way : is. You

问题 I have a question on pattern matching: Is it possible to somehow match a (string ++ [char] ++ anotherstring)? I have tried something like: f (s++";"++r) = s++r (the rhs is trivial, but its just for testing ;)) But this results in a parse error. 回答1: No, it's not possible. Pattern matching deconstructs values according to the constructors they were built with, so you can only use constructor applications in pattern matching to describe which values match the pattern and which don't. For

问题 I have a question on pattern matching: Is it possible to somehow match a (string ++ [char] ++ anotherstring)? I have tried something like: f (s++";"++r) = s++r (the rhs is trivial, but its just for testing ;)) But this results in a parse error. 回答1: No, it's not possible. Pattern matching deconstructs values according to the constructors they were built with, so you can only use constructor applications in pattern matching to describe which values match the pattern and which don't. For