Rust pattern matching with multiple types -


is possible pattern match in rust multiple types? seems though can't. if not, there planned support add or run-time type information (rtti)?

struct bus; struct car; struct person;  fn main() {     let x = bus;     //or more realistically, let x = function_with_multiple_return_types();      match x {         car => {             // ...         }         bus => {             // gets executed         }         person => {             // ...         }     } }

this example trivial. in real life, useful if x multiple types. e.g. let x = function_with_multiple_return_types();. shepmaster says, not possible because rust statically typed, unlike lisp.

no 1 can 100% accuracy feature or won't ever implemented, can 100% belief never implemented.

why that? because there's 0 benefit proposed syntax. rust statically-typed language. means compiler knows type of variable is. there's no way that branch besides bus ever executed. there's no way variable can have more 1 type! why language change allow add code never used? wouldn't useful.

a match statement, , pattern matching in general, useful when there multiple possible variants of something. that's why rust has enums; allow fixed set of dynamic choices (a.k.a. made @ runtime).

if need open set of dynamic decisions, that's traits (and maybe specialization) for. there's trait allows any concrete type.


Comments

Post a Comment

Popular posts from this blog

sql server - Cannot query correctly (MSSQL - PHP - JSON) -

i have little problem on project. , turns out query result returns nothing when value of first name or last name has 'ñ' in it. here's code config.php: <?php ini_set('mssql.charset', 'utf-8'); header('content-type: text/html; charset=utf-8'); $servername = "192.168.1.21"; /* ip add of db server */ $connection = array("database" => "db", "uid" => "?", "pwd" => "***"); $conn = sqlsrv_connect($servername, $connection); if(!$conn){ echo "connection not established."; die(print_r(sqlsrv_errors(), true)); } ?> myquery: $idnumber = $_post["idnum"]; $response = array(); $query2 = "select clname, cfname, cmname, cqualifier student cidno = '$idnumber'"; try{ $stmt2 = sqlsrv_query($conn, $query2); } catch(pdoexeption $ex){ $response["success"] = 0; $response["message"] = "d...
Read more

php - trouble displaying mysqli database results in correct order -

i have table in database stores events , date expire. want display events on website display current events first expired events after? i have tried following sql $sql = "select noticeid, notice, noticedesc, noticeimg, noticedate, expirydate tbl_notices group noticeid ,expired order expirydate asc"; but returns expired results first due expirydate being ordered asc in query. my tbl_notice structure looks this noticeid => int(4) notice => varchar(100) noticedesc => text noticedate => timestamp noticeimg => varchar(40) expirydate => datetime expired => int(1) 0 current or 1 expired my php code is $sql = "select noticeid, notice, noticedesc, noticeimg, noticedate, expirydate tbl_notices group noticeid ,expired order expirydate asc"; $result = mysqli_query($conn,$sql); if ($result->num_rows > 0) { // output data of each row while($row = $result->fetch_assoc()) { ...
Read more

C++ Linked List -

i trying create linked list download txt file , use linked list handle file line line. when handling downloaded linked list operations performed on such text editor would. encountering problems however. seems "node(string value)" section of code has wrong though original node() declaration no arguments passes. unable figure out quite is. node.h class node { public: node(); node(string value); void setnext(node *nextnode); // allows user set "next" pointer of node points friend class linkedlist; private: string data; // data box node* next; // pointer box }; node.cpp # include <string> # include "node.h" using namespace std; node::node() { data = ""; next = null; } node::node(string value) { data = value; next = null; } void node::setnext(node *nextnode) // allows user set "next" pointer of node points { this->next = nextnode; } your #include <string> ...
Read more