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
