[Issue 9817] New: Syntax change for front tuple expansion in foreach

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=9817

           Summary: Syntax change for front tuple expansion in foreach
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2013-03-26 06:04:17 PDT ---
I have tagged this as bug report instead of as enhancement request.

This program shows that D foreach is able to "unpack" tuples that are produced
by a range:


import std.algorithm: group;
void main() {
    auto data = ['a', 'a', 'b', 'b'];
    foreach (c, n; group(data)) {
        pragma(msg, typeof(c));
        pragma(msg, typeof(n));
    }
}


It prints:

dchar
uint


This feature is mostly undocumented:
http://d.puremagic.com/issues/show_bug.cgi?id=7361



But this feature doesn't work with an array of tuples (because it clashes
against a precedent semantics of foreach, that assigns the first variable to
the indexes of the array):


import std.typecons: tuple;
void main() {
    auto data = [tuple("red", 10), tuple("blue", 20)];
    foreach (x, y; data) {
        pragma(msg, typeof(x));
        pragma(msg, typeof(y));
    }
}



It prints:

uint
Tuple!(string, int)



In this thread (and on Bugzilla) I have suggested to add unpacking syntax in
various situations:

http://forum.dlang.org/post/gridjorxqlpoytuxwpsg@forum.dlang.org


I think the fully transparent unpacking syntax as in "foreach(c,n;
group(data))" is rigid (there is no way to tell if you want the first variable
to be bound to the array index, or to the first item of the tuple) and not
sufficiently explicit.

So I propose to replace the current syntax that unpacks tuples with a different
and explicit syntax that removed ambiguities, is more flexible, and works with
both arrays and ranges:


import std.algorithm: group;
import std.typecons: tuple;
void main() {
    auto data1 = ['a', 'a', 'b', 'b'];
    foreach ((c, n); group(data1)) {}
    auto data2 = [tuple("red", 10), tuple("blue", 20)];
    foreach ((s, n); data2) {}
    foreach (i, tup; data2) {}    
    foreach (i, (s, n); data2) {}    
}



This syntax is meant to be usable in other situations, so it must be designed
thinking about the future too:


import std.typecons: tuple;
void foo((string s, int b)) {}
void main() {
    foo(tuple("red", 10));
    auto (a, b) = tuple("red", 10);
    auto t = tuple("red", 10);
    switch (t) {
        case (a, b): break;
        default:
    }
}


Other syntaxes are possible.

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