Obtain predicate from SortedRange

[Ali Çehreli via Digitalmars-d-learn]

On 11/09/2016 02:02 AM, Jonathan M Davis via Digitalmars-d-learn wrote:
> On Wednesday, November 09, 2016 09:37:18 RazvanN via Digitalmars-d-learn
> wrote:
>> Given a SortedRange object, is there a way to obtain the
>> predicate which was used for it to be sorted?
>
> No. It only exists as a template argument to SortedRange and an alias within
> it, not as something that's actually part of the range object, and that
> alias is private. If you want access to the predicate, then you're going to
> have to pass something to sort or SortedRange which you can access
> separately from the SortedRange (e.g. an actual function rather than a
> lambda).
>
> - Jonathan M Davis
>

Once one knows that the predicate is the second template argument and 
what its semantics are, then one can hack their way through it. :)

import std.range;

import std.traits : isInstanceOf;
template predFun(alias R) {
     static if (is(R) &&
                isInstanceOf!(SortedRange, R)) {
         import std.functional : binaryFun;
         import std.traits : TemplateArgsOf;
         alias predFun = binaryFun!(TemplateArgsOf!R[1]);
     }
     else {
         alias predFun = predFun!(typeof(R));
     }
}

unittest {
     auto arr = [ 2, 1 ];
     auto r = assumeSorted!"a > b"(arr);
     alias func = predFun!r;
     assert(!func(10, 20));
     assert(func(100, 1));
}

unittest {
     auto arr = [ 1, 2, 3 ];
     auto r = assumeSorted!((a, b) => a < b)(arr);
     alias func = predFun!r;
     assert(func(2, 4));
     assert(!func(5, 3));
}

void main() {
}