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: [email protected]
ReportedBy: [email protected]
--- Comment #0 from [email protected] 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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