[Issue 6384] New: std.traits.isComparable

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=6384

           Summary: std.traits.isComparable
           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 2011-07-26 07:38:21 PDT ---
I suggest to add isComparable to std.traits, to be used as template constraint
for (both user defined and standard library) sorting/searching functions. It
looks for a opCmp or in general for


template isComparable(T) {
    enum bool isComparable = is(typeof({ return T.init < T.init; }));
}

version (unittest) {
    static assert(isComparable!int);
    static assert(isComparable!string);
    static assert(!isComparable!creal);

    static struct Foo {}
    static assert(!isComparable!Foo);

    static struct Bar {
        bool opCmp(Bar) { return true; }
    }
    static assert(isComparable!Bar);
}


I think using this constraint will improve the error messages of functions like
std.algorithm.sort too, etc:

import std.algorithm;
static struct Foo {}
static struct Bar {
    bool opCmp(Bar) { return true; }
}
void main() {
    auto a1 = [Foo(), Foo()];
    sort(a1);
    auto a2 = [Bar(), Bar()];
    sort(a2);
}


With DMD 2.054 it gives:

...\dmd\src\phobos\std\algorithm.d(6438): Error: static assert  "Invalid
predicate passed to sort: a < b"
test.d(8):        instantiated from here: sort!("a <
b",cast(SwapStrategy)0,Foo[])

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