http://d.puremagic.com/issues/show_bug.cgi?id=4579

           Summary: std.typecons.Tuple syntax unpacking sugar
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2010-08-03 17:13:16 PDT ---
In this enhancement request I propose to add syntax sugar to D2 that allows to "unpack" std.typecons.Tuple at the calling point.

This enhancement request doesn't ask for a third new kind of tuple, the normal std.typecons.Tuple Phobos2 one is enough.

Essentially std.typecons.Tuple can be used to have multiple return values in D, but some syntax sugar helps.


In Python2.x language, that is able to return tuples too, this automatic tuple unpacking is _very_ common and very handy (D makes this situation a little different, because std.typecons.Tuple supports named fields, and fields are statically typed):

def sqr_cube(x):
    return x*x, x*x*x
x2, x3 = sqr_cube(10)

(Python2.x also allows nested tuple unpacking with a bit of pattern matching, and Python3.x allows "variable length" unpacking with the * syntax.)


This is a function of std.file (Phobos2 of DMD 2.047):
void getTimes(in char[] name, out d_time ftc, out d_time fta, out d_time ftm);


Introducing tuples more into Phobos its signature becomes:
Tuple!(d_time "ftc", d_time "fta", d_time "ftm") getTimes(const string name);


But then you have to use it for example like this:

void main() {
    auto t3 = getTimes("filename");
    with(t3) {
        writeln(ftc, " ", fta, " ", ftm);
    }
}


While adding a bit of syntax sugar to D it becomes:

void main() {
    // Here I have used "fc" != "ftc"
    (d_time fc, d_time fa, d_time fm) = getTimes("filename");
}


That syntax sugar means something like:

void main() {
    auto __temp1 = getTimes("filename");
    d_time fc = __temp1.field[0];
    d_time fa = __temp1.field[1];
    d_time fm = __temp1.field[2];
}


I think that sugar doesn't clash with C syntax, so this syntax doesn't introduce bugs when porting code from C to D. This new syntax looks natural enough to me, and handy in high-level-style D programming.

Nested unpacking too is a possibility:

(int x, (int y, int z)) = foo();

See also bug 4577 for Tuples.

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--- Comment #1 from bearophile_hugs@eml.cc 2010-09-16 19:24:57 PDT ---
'auto' too may be supported:

void main() {
    (auto fc, auto fa, auto fm) = getTimes("filename");
}

Or even:

void main() {
    auto (fc, fa, fm) = getTimes("filename");
}


An unpacking syntax for Tuples is useful to replace the very similar zip() and
lockstep():


import std.algorithm, std.stdio, std.range;
void main() {
    foreach (p; zip([1, 2, 3], "abcd"))
        writeln(p._0, " ", p.length, " ", p._1);
    writeln();
    foreach (i, a, b; lockstep([1, 2, 3], "abcd"))
        writeln(i, " ", a, " ", b);
}


with a single zip() that may be used for both situations:

import std.algorithm, std.stdio, std.range;
void main() {
    foreach (p; zip([1, 2, 3], "abcd"))
        writeln(p[0], " ", p[1]);
    writeln();
    foreach ((a, b); zip([1, 2, 3], "abcd"))
        writeln(a, " ", b);
}


As in Python:

for p in zip([1, 2, 3], "abcd"):
    print p[0], p[1]
print
for (a, b) in zip([1, 2, 3], "abcd"):
    print a, b


(The zip() is a very commonly useful higher order function.)

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Andrei Alexandrescu <andrei@metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|                            |andrei@metalanguage.com
         AssignedTo|nobody@puremagic.com        |andrei@metalanguage.com


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--- Comment #2 from bearophile_hugs@eml.cc 2011-11-18 18:13:42 PST ---
See also issue 6365

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--- Comment #3 from bearophile_hugs@eml.cc 2012-02-29 14:10:39 PST ---
This syntax can't be used, it's part of the new lambda syntax:

auto tups = [tuple(1,2), tuple(3,4)];
auto r = map!((x,y) => x * y)(tups);


A possible solution:

auto tups = [tuple(1,2), tuple(3,4)];
auto r = map!(tuple(x,y) => x * y)(tups);

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