rust - Force/coerce evaluation of closure signature -


here contrived example of trying do:

use std::boxed::box;  #[derive(debug)] pub struct foo<'a>(pub &'a str);  pub trait intobox {     fn into_box<'a>(self) -> box<fn(foo) -> string>; }  impl<b> intobox b b: fn(foo) -> string + 'static {     fn into_box(self) -> box<fn(foo) -> string> { box::new(self) } }  fn direct_into_box<b: fn(foo) -> string + 'static>(b: b) -> box<fn(foo) -> string> {     box::new(b) }  fn main() {     // doesn't work     let x = intobox::into_box(|i| format!("{:?}", i) );      // works     let y = intobox::into_box(|i: foo| format!("{:?}", i) );      // works     let z = direct_into_box(|i| format!("{:?}", i) ); }

how trait impl same evaluation of closure done direct_into_box? have expected direct_into_box , trait impl behave in same way.

the error on x:

error[e0271]: type mismatch resolving `for<'r> <[closure@<anon>:20:31: 20:53] std::ops::fnonce<(foo<'r>,)>>::output == std::string::string`   --> <anon>:20:13    | 20 |     let x = intobox::into_box(|i| format!("{:?}", i) );    |             ^^^^^^^^^^^^^^^^^ expected bound lifetime parameter , found concrete lifetime    |    = note: concrete lifetime found lifetime '_#29r    = note: required because of requirements on impl of `intobox` `[closure@<anon>:20:31: 20:53]`    = note: required `intobox::into_box`  error[e0281]: type mismatch: type `[closure@<anon>:20:31: 20:53]` implements trait `std::ops::fn<(_,)>`, trait `for<'r> std::ops::fn<(foo<'r>,)>` required (expected concrete lifetime, found bound lifetime parameter )   --> <anon>:20:13    | 20 |     let x = intobox::into_box(|i| format!("{:?}", i) );    |             ^^^^^^^^^^^^^^^^^    |    = note: required because of requirements on impl of `intobox` `[closure@<anon>:20:31: 20:53]`    = note: required `intobox::into_box`

sounds an inference bug in compiler. seems happen compiler implements fn(foo<'x>) for 1 specific lifetime 'x instead of fn(foo<'a>) for lifetime 'a on closure.

let's see if can replicate error defining struct hand (this requires nightly compiler), can better understand what's going on. first, let's define struct correct way:

#![feature(fn_traits)] #![feature(unboxed_closures)]  // foo , intobox unchanged  struct func;  impl<'a> fnonce<(foo<'a>,)> func {     type output = string;      extern "rust-call" fn call_once(self, args: (foo<'a>,)) -> string {         self.call(args)     } }  impl<'a> fnmut<(foo<'a>,)> func {     extern "rust-call" fn call_mut(&mut self, args: (foo<'a>,)) -> string {         self.call(args)     } }  impl<'a> fn<(foo<'a>,)> func {     extern "rust-call" fn call(&self, (i,): (foo<'a>,)) -> string {         format!("{:?}", i)     } }  fn main() {     let x = intobox::into_box(func); }

this func struct compiles fine , behaves original closure.

now, let's break it:

impl fnonce<(foo<'static>,)> func {     type output = string;      extern "rust-call" fn call_once(self, args: (foo<'static>,)) -> string {         self.call(args)     } }  impl fnmut<(foo<'static>,)> func {     extern "rust-call" fn call_mut(&mut self, args: (foo<'static>,)) -> string {         self.call(args)     } }  impl fn<(foo<'static>,)> func {     extern "rust-call" fn call(&self, (i,): (foo<'static>,)) -> string {         format!("{:?}", i)     } }

what i've done here i've removed <'a> on each impl, impls no longer generic on lifetime, , i've replaced foo<'a> foo<'static>. means now, traits implemented when "closure"'s argument foo<'static>.

this fails compile following errors:

error[e0271]: type mismatch resolving `for<'r> <func std::ops::fnonce<(foo<'r>,)>>::output == std::string::string`   --> <anon>:51:13    | 51 |     let x = intobox::into_box(func);    |             ^^^^^^^^^^^^^^^^^ expected bound lifetime parameter , found concrete lifetime    |    = note: concrete lifetime found static lifetime    = note: required because of requirements on impl of `intobox` `func`    = note: required `intobox::into_box`  error[e0277]: trait bound `for<'r> func: std::ops::fn<(foo<'r>,)>` not satisfied   --> <anon>:51:13    | 51 |     let x = intobox::into_box(func);    |             ^^^^^^^^^^^^^^^^^ trait `for<'r> std::ops::fn<(foo<'r>,)>` not implemented `func`    |    = help: following implementations found:    = help:   <func std::ops::fn<(foo<'static>,)>>    = note: required because of requirements on impl of `intobox` `func`    = note: required `intobox::into_box`

the first error same, instead of internal name '_#29r, compiler mentions static lifetime, because that's used here. suspect compiler doing closure doesn't compile in code similar second set of impls, instead of 'static, it's other concrete lifetime can't name in rust. second error different means pretty same thing.


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