http://d.puremagic.com/issues/show_bug.cgi?id=6365

           Summary: AutoTupleDeclaration
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: k.hara.pg@gmail.com


--- Comment #0 from Kenji Hara <k.hara.pg@gmail.com> 2011-07-23 05:00:58 PDT ---
Syntax:
'auto' '(' Identifier [',' Identifier ...] ')' '=' Initializer ';'

Example:
auto (num, msg) = TypeTuple!(1, "hello");  // built-in tuple
assert(num == 1);
assert(msg == "hello");

auto (name, age) = tuple("John", 10);  // library tuple (= alias this tuple)
assert(name == "John");
assert(age == 10);

auto (n1, n2) = 100;  // non tuple initializer
assert(n1 == 100);
assert(n2 == 100);

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--- Comment #1 from Kenji Hara <k.hara.pg@gmail.com> 2011-07-23 05:08:51 PDT ---
Also we can allow a static array on initializer like follows:

auto (x, y) = [10, 20];
assert(x == 10);
assert(y == 20);

But, it has a bit problem.

auto (x, y) = [10, 20, 30];  // interpreted as non tuple initializer
assert(x == [10, 20, 30]);
assert(y == [10, 20, 30]);

auto (x, y) = [10];  // interpreted as non tuple initializer
assert(x == [10]);
assert(y == [10]);

It is not inconsistent, but less useful.

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bearophile_hugs@eml.cc


--- Comment #2 from bearophile_hugs@eml.cc 2011-07-23 05:14:44 PDT ---
(In reply to comment #0)

> auto (n1, n2) = 100;  // non tuple initializer
> assert(n1 == 100);
> assert(n2 == 100);

This *must* be a compile-time error.

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--- Comment #3 from Kenji Hara <k.hara.pg@gmail.com> 2011-07-23 05:17:57 PDT ---
(In reply to comment #2)
> (In reply to comment #0)
> 
> > auto (n1, n2) = 100;  // non tuple initializer
> > assert(n1 == 100);
> > assert(n2 == 100);
> 
> This *must* be a compile-time error.

This is keeping consistent feature.
Current D allow compilation of following code:

TypeTuple!(int, int) f = 10;
assert(f[0] == 10);
assert(f[1] == 10);

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--- Comment #4 from bearophile_hugs@eml.cc 2011-07-23 05:18:55 PDT ---
Optionally there is the foreach use case too. A possible syntax:

auto array = [tuple(1,"foo"), tuple(2,"bar")];
foreach (tuple(id, name); array) {}

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--- Comment #5 from bearophile_hugs@eml.cc 2011-07-23 05:23:06 PDT ---
(In reply to comment #3)

> This is keeping consistent feature.
> Current D allow compilation of following code:
> 
> TypeTuple!(int, int) f = 10;
> assert(f[0] == 10);
> assert(f[1] == 10);

I didn't know this, thank you. So the semantics of TypeTuple is broken. This design mistake must not be carried over to Tuples too. Consistancy with a so wrong design is like shooting yourself in the foot and makes this whole enhancement request of negative value.

It's better to fix this TypeTuple design bug instead. I will file an enhancement request on it.

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--- Comment #6 from Kenji Hara <k.hara.pg@gmail.com> 2011-07-23 05:27:14 PDT ---
(In reply to comment #4)
> Optionally there is the foreach use case too. A possible syntax:
> 
> auto array = [tuple(1,"foo"), tuple(2,"bar")];
> foreach (tuple(id, name); array) {}

I filed an issue about alias this + foreach range.front behavior.
http://d.puremagic.com/issues/show_bug.cgi?id=6366
Please comment there.

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Andrei Alexandrescu <andrei@metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrei@metalanguage.com


--- Comment #7 from Andrei Alexandrescu <andrei@metalanguage.com> 2011-07-23 07:32:23 PDT ---
One simple syntactic matter is that we can't extend the existing syntax to work with specified types. That means the user must use "auto" but cannot specify the types of the variables defined.

To allow that in the future, we need to move the paren before the auto:

(auto i, j) = tuple(1.2, "a");

Then the syntax can be later extended to:

(double i, string j) = tuple(1.2, "a");

Anyway, we should wait for Walter to weigh in. Thanks Kenji for this work.

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--- Comment #8 from Andrei Alexandrescu <andrei@metalanguage.com> 2011-07-23 07:33:33 PDT ---
Regarding mismatch in the number of elements, we may actually do good to disable

TypeTuple!(int, int) f = 10;
assert(f[0] == 10);
assert(f[1] == 10);

instead of striving to be consistent with it.

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--- Comment #9 from bearophile_hugs@eml.cc 2011-07-23 07:48:46 PDT ---
(In reply to comment #8)
> Regarding mismatch in the number of elements, we may actually do good to disable
> 
> TypeTuple!(int, int) f = 10;
> assert(f[0] == 10);
> assert(f[1] == 10);
> 
> instead of striving to be consistent with it.

This was right my point. On this I have opened bug 6367

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