Re: macros (was: Paul Graham's teaching style is bad}
From: Marco Parrone (marc0_at_autistici.org)
Date: 08/15/04
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Date: Sat, 14 Aug 2004 23:55:01 GMT
Bruce Hoult on Sun, 15 Aug 2004 10:49:15 +1200 writes:
> Oh, and btw, you didn't run your code before posting it.
Yes, I run it. I just had not known `repeat ... until'. I've never
seen it before, and I had misunderstood the description I found with a
little googling. I've run the code befor posting. All the code I
posted compile and run without errors nor warnings, running giving
meaningful output, with GCC 3.4.1.
>> /* repeat ... until: implementation #2: using GCC local labels */
>> /*
>> #define repeat { __label__ loop; loop:
>> #define until(cond) if (cond) goto loop; }
>> */
>>
>> /* repeat ... until: implementation #3: using GAS local labels */
>> #define repeat __asm__ ("1:");
>> #define until(cond) if (cond) __asm__ ("jmp 1b")
>
> Neither of these is going to handle nested loops.
Yes, the first nests. `myfor' nests too. I fixed the
`repeat...until' error.
#include <stdio.h>
#define repeat { __label__ loop; loop:
#define until(cond) if (! (cond)) goto loop; }
int
main (int argc, char *argv[])
{
int i;
int k;
k = 0;
repeat
i = 10;
printf ("[%d:", k);
repeat
printf (" %d", i);
--i;
until (i == 0);
printf ("]\n");
k++;
until (k == 10);
printf ("\n");
return 0;
}
marco@marc0:~/tmp$ gcc -Wall until.c
marco@marc0:~/tmp$ ./a.out
[0: 10 9 8 7 6 5 4 3 2 1]
[1: 10 9 8 7 6 5 4 3 2 1]
[2: 10 9 8 7 6 5 4 3 2 1]
[3: 10 9 8 7 6 5 4 3 2 1]
[4: 10 9 8 7 6 5 4 3 2 1]
[5: 10 9 8 7 6 5 4 3 2 1]
[6: 10 9 8 7 6 5 4 3 2 1]
[7: 10 9 8 7 6 5 4 3 2 1]
[8: 10 9 8 7 6 5 4 3 2 1]
[9: 10 9 8 7 6 5 4 3 2 1]
marco@marc0:~/tmp$
#include <stdio.h>
#define myfor(init, cond, step, body) { init; while (cond) { body; step; } }
int
main (int argc, char *argv[])
{
int i;
int j;
myfor (i = 0,
i < 10,
++i,
printf ("[%d:", i);
myfor (j = 10,
j > 0,
--j,
printf (" %d", j));
printf ("]\n"));
return 0;
}
marco@marc0:~/tmp$ gcc -Wall myfor.c
marco@marc0:~/tmp$ ./a.out
[0: 10 9 8 7 6 5 4 3 2 1]
[1: 10 9 8 7 6 5 4 3 2 1]
[2: 10 9 8 7 6 5 4 3 2 1]
[3: 10 9 8 7 6 5 4 3 2 1]
[4: 10 9 8 7 6 5 4 3 2 1]
[5: 10 9 8 7 6 5 4 3 2 1]
[6: 10 9 8 7 6 5 4 3 2 1]
[7: 10 9 8 7 6 5 4 3 2 1]
[8: 10 9 8 7 6 5 4 3 2 1]
[9: 10 9 8 7 6 5 4 3 2 1]
marco@marc0:~/tmp$
> All your examples have the same problem that while a programmer *can*
> correctly match up your new macros, there is no way to check that they
> have.
true, i just wanted to see if it can be done easily, not if it can be
done with checks and all that fancy things ;)
I find that the latest 2 challenges about walking the sources harder,
I don't think I can solve them, however I will try.
I like more the more explicit/detailed replies that I got in the
thread, than justing having to trust `this can not be done in C'.
> You also haven't shown how to write a C macro coresponding to the given
> example, that expands to arbitrarily nested for loops, with the depth
> depending on the way the user uses the macro.
Please let me see how it does not nest. Please note that the `myfor'
macro definition is not changed.
> Common Lisp and Dylan and Scheme macros actually create new syntax and
> integrate it into the language so seamlessly that you can't tell it
> isn't built in.
> That's no important when you're doing macros used to just one level, but
> when almost everything you do is a macro that expands to a macro that
> expands to a macro it is essential.
-- Marco Parrone <marc0@autistici.org> [0x45070AD6]
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