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Re: Tuples and other constant values

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2015年07月08日 19:38 GMT+02:00 Parke <parke.nexus@gmail.com>:
>> 2015年07月07日 0:13 GMT+02:00 Coda Highland <chighland@gmail.com>:
>>> A large proportion of the tuple users I've met are Python converts.
>
>> 2015年07月08日 2:34 GMT+02:00 Tim Hill <drtimhill@gmail.com>:
>>> You can think of them as an aggregate constant.
>
> On Wed, Jul 8, 2015 at 1:43 AM, Dirk Laurie <dirk.laurie@gmail.com> wrote:
>> The main reason why aggregate constants would be nice to have in
>> Lua is that more lurking bugs can be caught at compile time.
>
> How so? Can you provide an example that would result in a compile time error?
Back to my first post:
> Tuples are a special case of constant values. E.g.
>
> const c = {1,2,3}
>
> would create a constant table with three elements and associate it
> with the name c. The interpreter will remember that _ENV.c for this
> _ENV may not be redefined. A constant value can only be assigned
> to such constant table fields.
In the meantime I have realized, thanks to some postings in this thread,
that 'c', above, should be a local variable, not _ENV.c. The overhead in
tracing individual table fields would affect every key, but local variables
are not table entries.
If now:
c[2] = 1.5
the interpreter would know that `c` is const and generate "attempt to
change a constant object".
Similarly:
a = c
would be generate "attempt to assign a constant object to
a non-constant name", but
const a = c
would be OK.
It's quite similar to what happens in C. Try the following:
#include <stdio.h>
main() {
 typedef struct {
 double x, y;
 } complex;
 const complex I={0,1};
 I.x = 10;
 printf("%g %g\n",I.x,I.y);
}

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