[Issue 4577] New: Third way to create a std.typecons.Tuple

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=4577

           Summary: Third way to create a std.typecons.Tuple
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2010-08-03 15:02:23 PDT ---
Three ways to do something are a lot, but for me it's handy to build a
std.typecons.tuple with just field names, and using the type inference to omit
their types. With good library helper templates the implementation is simple:


import std.typecons: Tuple;
import std.string: split;
import std.metastrings: Format, toStringNow;

template SeriesGen1(string txt, string separator, int max, int min=0) {
    static if (min > max)
        const SeriesGen1 = "";
    else static if (min == max)
        const SeriesGen1 = Format!(txt, toStringNow!(max));
    else
        const SeriesGen1 = SeriesGen1!(txt, separator, max-1, min) ~ separator
~
                           Format!(txt, toStringNow!(max));
}

template SeriesGen2(string txt, string separator, int max, int min=0) {
    static if (min > max)
        const SeriesGen2 = "";
    else static if (min == max)
        const SeriesGen2 = Format!(txt, toStringNow!(max),
                                        toStringNow!(max));
    else
        const SeriesGen2 = SeriesGen2!(txt, separator, max-1, min) ~ separator
~
                           Format!(txt, toStringNow!(max),
                                        toStringNow!(max));
}

// not sure about the ref return value
// can this be named just "Tuple"?
ref auto Tuple_(string names, T...)(T args)
    if (T.length && split(names).length == T.length) {
    enum string[] fieldNames = split(names);

    mixin("return Tuple!(" ~
           SeriesGen2!("T[%s], fieldNames[%s]", ", ", T.length-1) ~
           ")(" ~
           SeriesGen1!("args[%s]", ", ", T.length-1) ~
           ");"
         );
}

// demo --------------------

import std.stdio: writeln;

ref auto sqr2(int a, float b) {
    return Tuple_!q{x y}(a * a, b * b);
}

void main() {
    ///*
    with(sqr2(10, 5.5F)) {
        writeln(x, " ", y);
    } // test.d(...): Error: sqr2(10,5.5F) is not an lvalue
    //*/

    auto xy = sqr2(10, 5.5F);
    with(xy) {
        writeln(x, " ", y);
    }

    with(Tuple!(int, "a", float, "b")(10, 5.5F)) {
        writeln(a, " ", b);
    }
}


I'd like this to overload the std.typecons.Tuple, I hope this is possible.

I am not sure if this enhancement request deserves a 'patch' keyword tag.

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