Explain function syntax

[Paul Backus]

On Wednesday, 17 September 2025 at 22:16:50 UTC, Brother Bill 
wrote:
> Then there is another example with 'delegate' instead of 
> 'function.
>
> ```
> import std.stdio;
>
> void main()
> {
>
> }
>
> alias Calculator = int function(int);
>
> Calculator makeCalculator()
> {
> 	int increment = 10;
> 	return value => increment + value; // ← compilation ERROR
> }
>
> ```

[Nested functions][1] that access local variables from their 
enclosing function need to take an additional, hidden "context" 
parameter in order to actually find those variables at runtime. 
Because of this, they have a different calling convention than 
"normal" functions. As a consequence, if you take a pointer to 
one of these functions, you do not get a normal "function 
pointer" type (which is represented by the `function` keyword), 
but a special ["delegate" type][2], which contains both a pointer 
to the function itself, and a pointer to the context needed to 
call it successfully.

[1]: https://dlang.org/spec/function.html#nested
[2]: https://dlang.org/spec/function.html#closures