XIEITE_FN(), XIEITE_FN_THIS(), XIEITE_FN_LOCAL(), XIEITE_FN_MUT()

 Defined in header <xieite/pp/fn.hpp>

Allows a shorthand lambda syntax:
 XIEITE_FN($0 + $1)
which expands to something like:
 []<typename...>(auto&& a, auto&& b) noexcept(noexcept(a + b)) -> decltype(auto) requires(requires { a + b; }) { return a + b; }

The body of a shorthand lambda has access to a parameter pack $ containing all arguments, a template parameter pack $$ containing all template arguments, variables $0 through $255 which correspond to individual arguments, and types $$0 through $$255 which correspond to individual template arguments.
Accessing a variable or type which does not correspond to an argument passed to the lambda (when the number of arguments is not greater than the variable's index or the number of template arguments is not greater than the type's index) results in some ambiguously unusable value.


Definitions

 #fn 
#define XIEITE_FN(...) /* ([](???) static ??? { ??? }) */
Expands to a lambda expression with no capture-default and a static specifier.

#fn-this 
#define XIEITE_FN_THIS(...) /* ([](this auto, ???) ??? { ??? }) */
Expands to a lambda expression with no captures, no specifiers, and an explicit this parameter through variable $this of type $$this.

#fn-local 
#define XIEITE_FN_LOCAL(captures_, ...) /* ([???](this auto, ???) ??? { ??? }) */
Expands to a lambda expression with user-provided captures, no specifiers, and an explicit this parameter through variable $this of type $$this. captures_ must be wrapped in parentheses if more than one capture item is passed, otherwise parentheses are optional.

#fn-mut 
#define XIEITE_FN_MUT(captures_, ...) /* ([???](this auto, ???) mutable ??? { ??? }) */
Expands to a lambda expression with user-provided captures, a mutable specifier, and an explicit this parameter through variable $this of type $$this. captures_ must be wrapped in parentheses if more than one capture item is passed, otherwise parentheses are optional.


Examples

 #0 
int main() {
	auto add_two = XIEITE_FN($0 + $1);
	auto add_all = XIEITE_FN((... + $));
	auto cast = XIEITE_FN(static_cast<$$0>($0));

	xieite::dump(add_two(3, 4));
	xieite::dump(add_all(1, 2, 3, 4, 5));
	xieite::dump(cast.operator()<int>(6.28318));
}
Output: 
7
15
6
[View in Compiler Explorer]

#1 
int main() {
	struct : decltype(XIEITE_FN_THIS($this.x + $0)) { int x = 5; } foo;

	xieite::dump(foo(1));
}
Output: 
6
[View in Compiler Explorer]

#2 
int main() {
	int x = 5;
	auto foo = XIEITE_FN_LOCAL(=, x * $0);

	xieite::dump(foo(2));
}
Output: 
10
[View in Compiler Explorer]

#3 
int main() {
	auto foo = XIEITE_FN_MUT(x = 0, x++);
	for (int i : { foo(), foo(), foo() }) {
		xieite::dump(i);
	}
}
Output: 
0
1
2
[View in Compiler Explorer]