What is the ~> (tilde greater than) operator used for in Swift? -

-

swift 1.1 includes declaration of ~> operator:

infix operator ~> {     associativity left     precedence 255 }

what used in swift? appears declared no functions defined leverage it. other developers have used reactive patterns , marshaling closures between queues, wondering why it's defined in standard framework. surmise it's there "reserve" custom operator developer use, given has highest precedence possible.

since swift has been open-sourced can see actual reason including ~> in stdlib: as workaround method specialization in child protocol in swift 1.x.

// workaround <rdar://problem/14011860> subtlf: default // implementations in protocols.  library authors should ensure // operator never needs seen end-users.  see // test/prototypes/genericdispatch.swift documented // example of how operator used, , how use can hidden // users. infix operator ~> { associativity left precedence 255 }

a detailed example can found in test/prototypes/genericdispatch.swift.

note: do not use ~> in swift 2+. historical workaround. no longer needed. read on.

how ~> works

in swift 2, remaining instance of ~> abs function (which going away well). can see how ~> works. stdlib/public/core/integerarithmetic.swift.gyb, signedinteger protocol defines ~> operator:

struct _abs {}  protocol signednumber: comparable {     prefix func -(_: self) -> self      func ~> (_: self, _: (_abs, ()) -> self }  func ~> <t: signednumber>(x: t, _: (_abs, ()) -> t {     return x < 0 ? -x : x }

here, rhs of arrow ~> specifies command can sent object. think of p~>(cmd, args) p.cmd(args).

then provide default implementation of _abs when given signednumber. 

protocol absolutevaluable : signednumber {     static func abs(_: self) -> self }  func ~> <t: absolutevaluable>(x: t, _: (_abs, ())) -> t {     return t.abs(x) }

next, have child protocol, absolutevaluable, perhaps has more efficient way compute absolute value x < 0 ? -x : x. specialize ~> operator absolutevaluable.

func abs<t: signednumber>(_ x: t) -> t {     return x ~> (_abs(), ()) }

finally hide ~> call in public wrapper method. if t absolutevaluable, more specialized , more efficient ~> chosen.

this why 42 ~> _advance(12) or 42 ~> _distanceto(23) shown in @rintaro's answer, because .advanceby , .distanceto methods o(n) general forwardindextype, can implemented in o(1) if type randomaccessindextype.

you don't need ~>

this pattern done without invoking ~> operator, using extensions on protocol:

protocol signedinteger: comparable {     prefix func -(_: self) -> self      func genericabs() -> self }  extension signedinteger {     func genericabs() -> self {         return self < 0 ? -self : self     } }  protocol absolutevalueable: signedinteger {     static func abs(_: self) -> self }  extension absolutevalueable {     func genericabs() -> self {         return self.abs(self)     }     // allow subtypes override     // genericabs() directly, instead of overriding      // static abs(). }  func abs<t: signedinteger>(x: t) -> t {     return x.genericabs() }

in particular why other ~> implementations besides abs gone in swift 2: specializations using technique has been changed use protocol extensions more obvious, e.g.

note: radar problem 14011860 not public, may see scope of bug duplicates on openradar:


Comments

Post a Comment

Popular posts from this blog

c++ - No viable overloaded operator for references a map -

-
i trying use map can make tag names reference number. when try , use it, in code error (6 in total every time reference map): src/main.cpp:25:45: error: no viable overloaded operator[] type 'std::map<std::string, std::string>' const char* idcs = node.child_value(tagmap[3]); this code: #include "pugi/pugixml.hpp" #include <iostream> #include <string> #include <map> int main() { pugi::xml_document doca, docb; std::map<std::string, pugi::xml_node> mapa, mapb; std::map<std::string, std::string> tagmap {{"1", "data"}, {"2", "entry"}, {"3", "id"}, {"4", "content"}}; if (!doca.load_file("a.xml") || !docb.load_file("b.xml")) { std::cout << "can't find input files"; return 1; } (auto& node: doca.child(tagmap[1]).children(tagmap[2])) { co
Read more

java - Custom OutputStreamAppender not run: LOGBACK: No context given for <MYAPPENDER> -

-
i wish write custom appender on basis of outputstreamappender . wrote following class package tests; import java.io.ioexception; import java.io.outputstream; import ch.qos.logback.core.outputstreamappender; public class myappender<e> extends outputstreamappender<e> { public myappender() { system.out.println("myappender created"); setoutputstream(new outputstream() { @override public void write(int b) throws ioexception { throw new unsupportedoperationexception(); } }); } } then wrote following runner: package tests; import org.slf4j.loggerfactory; import ch.qos.logback.classic.logger; public class runner { private static final logger log = (logger) loggerfactory.getlogger(runner.class); public static void main(string[] args) { //log.addappender(new myappender<iloggingevent>()); log.info("hello world"); } } finall
Read more

java - Cannot secure connection using TLS -

-
i'm trying establish secure socket connection between java client applet (built jdk 1.7.0_75-b13) , vc++ server application. as test vehicle, used vc++ client/server sample found in msdn forums, modified use schannel , able establish socket using cipher suite tls_rsa_with_aes_128_cbc_sha. works of tls 1.0/1.1/1.2. when try opening socket java applet same server application, connection rejected server reporting following: tls 1.0 acceptsecuritycontext failed: 0x80090327 tls 1.1 acceptsecuritycontext failed: 0x80090331 tls 1.2 acceptsecuritycontext failed: 0x80090331 this java code used create socket: debugprint("setting secure connection"); sslsocketfactory sslsocketfactory = (sslsocketfactory) sslsocketfactory.getdefault(); sslsocket sslsocket = (sslsocket) sslsocketfactory.createsocket("127.0.0.1", socketnumber); debugprint("starting handshake"); sslsocket.settcpnodelay(true); sslsocket.setsolinger(false, 0); sslsocket.s
Read more