Binary search in structs

[FreeSlave via Digitalmars-d-learn]

I have array of structs sorted by specific field. How can I 
perform binary search using this field as a key?

Currently I ended up with this, but it gives error:
struct S
{
    int i;
    string s;
}

import std.range;

void main(string [] args)
{
    S[] structs = [{1,"hello"}, {2,"world"}, {3, "!"}]; //sorted 
by i

    auto sortedRange = assumeSorted!(function bool(ref const S 
item, int needle) {
        return item.i < needle;
    })(structs);

    sortedRange.trisect(2); //compilation error
}