How to work around the infamous dual-context when using delegates together with std.parallelism

[sighoya]

On Thursday, 27 May 2021 at 09:58:40 UTC, Christian Köstlin wrote:
> I have this small program here
>
> test.d:
> ```
> import std;
> string doSomething(string[] servers, string user) {
>     return user ~ servers[0];
> }
> void main() {
>     auto servers = ["s1", "s2", "s3"];
>     auto users = ["u1", "u2", "u3"];
>     writeln(map!(user => servers.doSomething(user))(users));
>     writeln(taskPool.amap!(user => 
> servers.doSomething(user))(users));
> }
> ```



I think it relates to 
https://issues.dlang.org/show_bug.cgi?id=5710

The reason is that amap requires a this pointer of type TaskPool 
and a context pointer to the closure which belongs to main, at 
least because it requires servers. Having both isn't possible due 
to problems in non DMD compilers.

If you rewrite it more statically:
```D
string doSomething(string[] servers, string user) {
     return user ~ servers[0];
}

string closure(string user)
{
     return servers.doSomething(user);
}
auto servers = ["s1", "s2", "s3"];
int main()
{
     auto users = ["u1", "u2", "u3"];
     writeln(map!(user => servers.doSomething(user))(users));
     writeln(taskPool.amap!(closure)(users));
     return 0;
}
```

PS: Just enable markdown if you want to highlight D code