[Issue 11913] New: nothrow std.algorithm.zip

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=11913

           Summary: nothrow std.algorithm.zip
           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 2014-01-13 04:44:56 PST ---
void main() nothrow {
    import std.algorithm: zip;
    int[] a;
    foreach (p; zip(a, a)) {}
}



dmd 2.065alpha gives:

temp.d(4): Error: 'std.range.Zip!(int[], int[]).Zip.empty' is not nothrow
temp.d(4): Error: 'std.range.Zip!(int[], int[]).Zip.popFront' is not nothrow
temp.d(1): Error: function 'D main' is nothrow yet may throw


The signature of zip is:

auto zip(Ranges...)(Ranges ranges) if (Ranges.length &&
allSatisfy!(isInputRange, Ranges));

auto zip(Ranges...)(StoppingPolicy sp, Ranges ranges) if (Ranges.length &&
allSatisfy!(isInputRange, Ranges)); 


The empty() method of Zip:

@property bool empty() {
    import std.exception : enforce;

    final switch (stoppingPolicy) {
        case StoppingPolicy.shortest:
            foreach (i, Unused; R)
                if (ranges[i].empty) return true;
            return false;
        case StoppingPolicy.longest:
            foreach (i, Unused; R)
                if (!ranges[i].empty) return false;
            return true;
        case StoppingPolicy.requireSameLength:
            foreach (i, Unused; R[1 .. $])
                enforce(ranges[0].empty ==
                        ranges[i + 1].empty,
                        "Inequal-length ranges passed to Zip");
            return ranges[0].empty;
    }
    assert(false);
}


The case StoppingPolicy.requireSameLength can throw, so zip can't be nothrow.

But with a different API:

auto zip(StoppingPolicy = StoppingPolicy.requireSameLength, Ranges...)(Ranges
ranges)

You can use a "static if" inside Zip to tell apart the various cases (because D
doesn't have a static switch), allowing most times zip to be usable inside
nothrow functions.

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