www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - What is a sink delegate?

reply "Gary Willoughby" <dev nomad.so> writes:
What is a sink delegate?

Discussed here:
http://forum.dlang.org/thread/m0bdgg$1t7j$1 digitalmars.com?page=6#post-m0emvc:242av5:241:40digitalmars.com
Sep 30 2014
next sibling parent reply "Adam D. Ruppe" <destructionator gmail.com> writes:
On Tuesday, 30 September 2014 at 17:22:44 UTC, Gary Willoughby 
wrote:

 What is a sink delegate?


Instead of

string toString() { return "foo"; }

for example, you would use:

void toString(void delegate(string) sink) { sink("foo"); }

The sink argument there is then free to view and discard the data 
or to make a private copy using whatever scheme it desires.
Sep 30 2014
parent reply "Joel" <joelcnz gmail.com> writes:
On Tuesday, 30 September 2014 at 17:27:09 UTC, Adam D. Ruppe 
wrote:

 On Tuesday, 30 September 2014 at 17:22:44 UTC, Gary Willoughby 
 wrote:

 What is a sink delegate?


 Instead of

 string toString() { return "foo"; }

 for example, you would use:

 void toString(void delegate(string) sink) { sink("foo"); }

 The sink argument there is then free to view and discard the 
 data or to make a private copy using whatever scheme it desires.



How do you use that toString? Maybe an example? Below is my 
failed effort.

import std.stdio;

struct Try {
	string name;
	long age;
	
	void toString(void delegate(string) sink) {
		sink("foo");
	}
}

void main() {
	Try t = Try("Joel", 35);
	writeln(t);
}
Oct 09 2014
next sibling parent reply "thedeemon" <dlang thedeemon.com> writes:
On Friday, 10 October 2014 at 03:06:33 UTC, Joel wrote:


 How do you use that toString? Maybe an example?


void main() {
  	Try t = Try("Joel", 35);
	t.toString(s => writeln(s));
}
Oct 09 2014
next sibling parent reply "Hjkp" <Hjkp nowhere.me> writes:
On Friday, 10 October 2014 at 04:42:04 UTC, thedeemon wrote:

 On Friday, 10 October 2014 at 03:06:33 UTC, Joel wrote:


 How do you use that toString? Maybe an example?


 void main() {
  	Try t = Try("Joel", 35);
 	t.toString(s => writeln(s));
 }


I think that the problem pointed by the OP is that toString is 
usually used automatically for struct in std.conv when it's 
implemented. I'm myself quite dubitatif in front of this syntax, 
even if IIRC during one talk of the DConf 2014 someone claim 
that's it's faster.
Oct 10 2014
parent "VaZyJeanPierre" <VaZyJeanPierre memelevelo.fr> writes:
On Friday, 10 October 2014 at 11:35:21 UTC, Hjkp wrote:

 On Friday, 10 October 2014 at 04:42:04 UTC, thedeemon wrote:

 On Friday, 10 October 2014 at 03:06:33 UTC, Joel wrote:


 How do you use that toString? Maybe an example?


 void main() {
 	Try t = Try("Joel", 35);
 	t.toString(s => writeln(s));
 }


 I think that the problem pointed by the OP is that toString is 
 usually used automatically for struct in std.conv when it's 
 implemented. I'm myself quite dubitatif in front of this 
 syntax, even if IIRC during one talk of the DConf 2014 someone 
 claim that's it's faster.


I think it was in

http://dconf.org/2014/talks/strasuns.html
Oct 10 2014
prev sibling parent "Joel" <joelcnz gmail.com> writes:
Thanks thedeemon.

On Friday, 10 October 2014 at 04:42:04 UTC, thedeemon wrote:

 On Friday, 10 October 2014 at 03:06:33 UTC, Joel wrote:


 How do you use that toString? Maybe an example?


 void main() {
  	Try t = Try("Joel", 35);
 	t.toString(s => writeln(s));
 }
Oct 10 2014
prev sibling parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 10/09/2014 08:06 PM, Joel wrote:

 On Tuesday, 30 September 2014 at 17:27:09 UTC, Adam D. Ruppe wrote:

 On Tuesday, 30 September 2014 at 17:22:44 UTC, Gary Willoughby wrote:

 What is a sink delegate?


 Instead of

 string toString() { return "foo"; }

 for example, you would use:

 void toString(void delegate(string) sink) { sink("foo"); }

 The sink argument there is then free to view and discard the data or
 to make a private copy using whatever scheme it desires.



 How do you use that toString? Maybe an example? Below is my failed effort.

 import std.stdio;

 struct Try {
      string name;
      long age;

      void toString(void delegate(string) sink) {
          sink("foo");
      }
 }

 void main() {
      Try t = Try("Joel", 35);
      writeln(t);
 }


The signature of that toString is different from what I have been seeing 
and using. The following works:

     void toString(void delegate(const(char)[]) sink) const {

Ali
Oct 09 2014
parent reply Steven Schveighoffer <schveiguy yahoo.com> writes:
On 10/10/14 1:00 AM, Ali Çehreli wrote:

 On 10/09/2014 08:06 PM, Joel wrote:

 On Tuesday, 30 September 2014 at 17:27:09 UTC, Adam D. Ruppe wrote:

 On Tuesday, 30 September 2014 at 17:22:44 UTC, Gary Willoughby wrote:

 What is a sink delegate?


 Instead of

 string toString() { return "foo"; }

 for example, you would use:

 void toString(void delegate(string) sink) { sink("foo"); }

 The sink argument there is then free to view and discard the data or
 to make a private copy using whatever scheme it desires.



 How do you use that toString? Maybe an example? Below is my failed
 effort.

 import std.stdio;

 struct Try {
      string name;
      long age;

      void toString(void delegate(string) sink) {
          sink("foo");
      }
 }

 void main() {
      Try t = Try("Joel", 35);
      writeln(t);
 }


 The signature of that toString is different from what I have been seeing
 and using. The following works:

      void toString(void delegate(const(char)[]) sink) const {


The delegate parameter is what is important. The function that is going 
to be passed in takes a const(char)[], which actually should, but does 
not, implicitly cast to a delegate(string) (see issue 
https://issues.dlang.org/show_bug.cgi?id=3075).

The const outside is irrelevant to whether it will accept it or not, 
that is a contract between the toString function and your object. If you 
want a non-const toString, I think that should work.

(actually, testing it...) Yep, it works without the const on the outside.

-Steve
Oct 10 2014
parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 10/10/2014 06:30 AM, Steven Schveighoffer wrote:


 On 10/10/14 1:00 AM, Ali Çehreli wrote:

 On 10/09/2014 08:06 PM, Joel wrote:

 On Tuesday, 30 September 2014 at 17:27:09 UTC, Adam D. Ruppe wrote:

 On Tuesday, 30 September 2014 at 17:22:44 UTC, Gary Willoughby wrote:

 What is a sink delegate?


 Instead of

 string toString() { return "foo"; }

 for example, you would use:

 void toString(void delegate(string) sink) { sink("foo"); }

 The sink argument there is then free to view and discard the data or
 to make a private copy using whatever scheme it desires.



 How do you use that toString? Maybe an example? Below is my failed
 effort.

 import std.stdio;

 struct Try {
      string name;
      long age;

      void toString(void delegate(string) sink) {
          sink("foo");
      }
 }

 void main() {
      Try t = Try("Joel", 35);
      writeln(t);
 }


 The signature of that toString is different from what I have been seeing
 and using. The following works:

      void toString(void delegate(const(char)[]) sink) const {


 The delegate parameter is what is important. The function that is going
 to be passed in takes a const(char)[],


That is what I meant.


 which actually should, but does
 not, implicitly cast to a delegate(string) (see issue
 https://issues.dlang.org/show_bug.cgi?id=3075).

 The const outside is irrelevant to whether it will accept it or not,
 that is a contract between the toString function and your object. If you
 want a non-const toString, I think that should work.

 (actually, testing it...) Yep, it works without the const on the outside.


But not for const objects. The following program does not call the user 
defined toString:

import std.stdio;
import std.conv;

struct S
{
     int i;

     void toString(void delegate(const(char)[]) sink)
     {
         sink(i.to!string);
     }
}

void main()
{
     const c = S(42);
     writeln(c);
}

Add a const at the end, now it calls the user defined one.


 -Steve


Ali
Oct 10 2014
parent Steven Schveighoffer <schveiguy yahoo.com> writes:
On 10/10/14 11:20 AM, Ali Çehreli wrote:

 On 10/10/2014 06:30 AM, Steven Schveighoffer wrote:

  > The const outside is irrelevant to whether it will accept it or not,
  > that is a contract between the toString function and your object. If you
  > want a non-const toString, I think that should work.
  >
  > (actually, testing it...) Yep, it works without the const on the
 outside.

 But not for const objects.


I think that's what I said :) It's a contract between the toString 
function and your object, it has nothing to do with writeln accepting 
the function. There are some quirky requirements for certain "magic" 
functions in phobos that have to be exactly a certain signature for the 
compiler to use it.

Now, obviously, the toy example can be labeled const. But not one that 
might, say, cache some state in order to compute the output.


 The following program does not call the user
 defined toString:

 import std.stdio;
 import std.conv;

 struct S
 {
      int i;

      void toString(void delegate(const(char)[]) sink)
      {
          sink(i.to!string);


Don't do this. Do this instead:

import std.format;
sink.formattedWrite(i);

The former allocates memory on the heap, just to throw it away. You are 
completely losing the benefit of the sink.

-Steve
Oct 10 2014
prev sibling parent Steven Schveighoffer <schveiguy yahoo.com> writes:
On 9/30/14 1:22 PM, Gary Willoughby wrote:

 What is a sink delegate?

 Discussed here:
 http://forum.dlang.org/thread/m0bdgg$1t7j$1 digitalmars.com?page=6#post-m0emvc:242av5:241:40digitalmars.com


Aside from Adam's answer, the term 'sink' means to draw out something, 
as in 'heat sink'. So basically a sink delegate is a place to put the 
string data.

-Steve
Sep 30 2014