swap() macro

[Chris Torek]

In article <1836 at brl-smoke.ARPA> gwyn at brl.arpa (Doug Gwyn
(VLD/VMB) <gwyn>) writes:
>It may be amusing and/or instructive to contemplate the fact that
>there is no way to write a function that exchanges the contents of
>two variables in a language where parameters are passed "by name".

How so?  It seems rather simple.  I have here a C program that effects
call-by-name and does indeed perform a swap:

/*
 * Call by name example of swap.
 *
 * Call by name is done by passing `thunks', where a `thunk' is a
 * function that returns the address of an argument.  In this case
 * calling the function provided via `f1' returns the address of
 * the first argument; indirecting through this address produces
 * the argument itself, as an lvalue (i.e., `named').  Similarly,
 * indirection through f2's return value names the second argument.
 */
swap(f1, f2)
	int *(*f1)(), *(*f2)();
	/* `pointer to function returning pointer to int' */
{
	int t;
	
	t = *(*f1)();
	*(*f1)() = *(*f2)();
	*(*f2)() = t;
}

/*
 * Here are the variables we will address for swap().
 */
int	a, b;

/*
 * Here are the two `thunk' functions.
 */
int *
addr_a()
{

	return (&a);
}

int *
addr_b()
{

	return (&b);
}

/*
 * Finally, demonstrate that swap() does indeed work:
 */
/*ARGSUSED*/
main(argc, argv)
	int argc;
	char **argv;
{

	a = 3;
	b = 7;
	swap(addr_a, addr_b);
	printf("should be 7, 3: a = %d, b = %d\n", a, b);
	exit(0);
}

/*
 * Incidentally, `swap' can be passed a thunk that names an expression;
 * here is what a call-by-name compiler might generate for `a+b':
 *
 * int *
 * addr_aplusb()
 * {
 *	int t = a + b;
 *
 *	return (&t);
 * }
 */
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 1516)
UUCP:	seismo!umcp-cs!chris
CSNet:	chris at umcp-cs		ARPA:	chris at mimsy.umd.edu