I can't understand how to get works delegates works from this doc http://vibed.org/api/vibe.http.client/requestHTTP

I see example, but when I am looking on Prototypes I really can't figure how to use them.

For example what does this mean:
 scope void delegate(scope HTTPClientRequest) requester,

it's function that point to method inside class? Why I can not call it like:

requestHTTP(url,
	scope req,
	scope res);

I am getting error:

Error: expression expected, not 'scope'
Error: found 'req' when expecting ','
Error: expression expected, not ','
Error: found 'scope' when expecting ','

---
Is it's optional parameter?

scope void delegate(scope HTTPClientRequest) requester = cast(void delegate(scope HTTPClientRequest req))null

so I can place instead in just `null`?

On Wednesday, 16 March 2016 at 20:08:40 UTC, Suliman wrote:
> I can't understand how to get works delegates works from this doc http://vibed.org/api/vibe.http.client/requestHTTP
>
> I see example, but when I am looking on Prototypes I really can't figure how to use them.
>
> For example what does this mean:
>  scope void delegate(scope HTTPClientRequest) requester,
>
> it's function that point to method inside class?

No, its a delegate, essentially a fat pointer bundling a function and a context together.
It can be a pointer to a class member, e.g. `&myHandler.sendHttpRequest`, but it can also
be a literal using the context of the function:

```
void foo ()
{
  string foo;

  scope requester = (scope HTTPClientRequest req) { req.header["foo"] = foo; }

  requestHTTP("http://google.com", requester);
}
```

By default, `delegate`s are allocated on the heap (GC-allocated).
`scope` delegate means its allocated on the stack (which also means that if you return a `scope delegate`, you'll get random behaviour / segfault because your context points to an invalidated stack frame).


> Why I can not call it like:
>
> requestHTTP(url,
> 	scope req,
> 	scope res);
>
> I am getting error:
>
> Error: expression expected, not 'scope'
> Error: found 'req' when expecting ','
> Error: expression expected, not ','
> Error: found 'scope' when expecting ','
>
> ---

You have two usage of `scope` here. First a `scope delegate` which is used to avoid memory allocation. This `scope delegate` takes parameter: the requester takes a request object (what will be send to the server), and the receiver takes a response object (what has been read from the connection as the response). Those object are marked as `scope`, which means the memory will be destroyed when you exit the `delegate`s. That's also to reduce memory allocation. If you want to save anything (like some headers value), you have to [i]dup it.

> Is it's optional parameter?
>
> scope void delegate(scope HTTPClientRequest) requester = cast(void delegate(scope HTTPClientRequest req))null
>
> so I can place instead in just `null`?

Yes. The reason for the various overloads is performance vs convenience.
If you provide both requester and receiver, you can process request with a minimal amount of memory allocation. However, if you take the overload that returns an `HTTPClientResponse`, then it will have to allocate it (as well as all the data inside).

Thanks! I am understand a little bit better, but not all.

```
shared static this()
{
	auto settings = new HTTPServerSettings;
	settings.port = 8080;

	listenHTTP(settings, &handleRequest);
}

void handleRequest(HTTPServerRequest req,
                   HTTPServerResponse res)
{
	if (req.path == "/")
		res.writeBody("Hello, World!", "text/plain");
}
```
https://github.com/rejectedsoftware/vibe.d/blob/master/source/vibe/http/server.d#L104
I expected to see in listenHTTP() function some processing but it's simply get args and then do return:
return listenHTTP(...)

Could you explain why it's do so?

--
Where is constructor of this class? http://vibed.org/api/vibe.http.client/HTTPClientRequest

How I should use it if have only docs, and do not have examples? (I am trying understand how to use docs without cope-past examples)

On Saturday, 19 March 2016 at 19:53:01 UTC, Suliman wrote:
> Thanks! I am understand a little bit better, but not all.
>
> ```
> shared static this()
> {
> 	auto settings = new HTTPServerSettings;
> 	settings.port = 8080;
>
> 	listenHTTP(settings, &handleRequest);
> }
>
> void handleRequest(HTTPServerRequest req,
>                    HTTPServerResponse res)
> {
> 	if (req.path == "/")
> 		res.writeBody("Hello, World!", "text/plain");
> }
> ```
> https://github.com/rejectedsoftware/vibe.d/blob/master/source/vibe/http/server.d#L104
> I expected to see in listenHTTP() function some processing but it's simply get args and then do return:
> return listenHTTP(...)
>
> Could you explain why it's do so?
>
> --
> Where is constructor of this class? http://vibed.org/api/vibe.http.client/HTTPClientRequest
>
> How I should use it if have only docs, and do not have examples? (I am trying understand how to use docs without cope-past examples)

The function is overloaded; it's calling the main implementation at line 77 after transforming the arguments.

The constructor for HTTPClientRequest is likely undocumented because you should not construct it yourself; vibe.d constructs it and passes it to the function you register with listenHTTP.

> The constructor for HTTPClientRequest is likely undocumented because you should not construct it yourself; vibe.d constructs it and passes it to the function you register with listenHTTP.

How to understand looking at docs that it work as you saying?