Re: 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() {
}