Re: Trying to use a template class with ranges

[mark via Digitalmars-d-learn]

On Thursday, 6 February 2020 at 15:21:46 UTC, Steven 
Schveighoffer wrote:
[snip]
3. You should declare constraints signifying what types are 
valid. i.e.:

class Diff(T) if (
   isForwardRange!T // it's a forward range
   && is(typeof(T.init.front == T.init.front)) // elements are 
comparable
)

Might be good to put this constraint on the factory function 
too.

I don't know how to do that syntactically. I tried the obvious 
and it wouldn't compile.

But even without that it won't compile although it is much closer 
now!

import std.range: ElementType, front, isForwardRange;

class Diff(T) if (
        isForwardRange!T && // T is a range
        is(typeof(T.init.front == T.init.front)) // Elements 
support ==
        ) {
    alias E = ElementType!T;

    T a;
    T b;
    size_t[][E] b2j;

    this(T a, T b) {
        this.a = a;
        this.b = b;
        chainB();
    }

    private final void chainB() {
        foreach (i, element; b)
            b2j[element] ~= i;
        // TODO
    }
}

auto differ(T)(T a, T b) { return Diff!T(a, b); }

unittest {
    import std.array;
    import std.stdio: writeln;

    writeln("unittest for the diffrange library.");
    auto d1 = differ("one two three four".array, "one too tree 
four".array);
    auto a = ["Tulips are yellow,", "Violets are blue,", "Agar is 
sweet,",
              "As are you."];
    auto b = ["Roses are red,", "Violets are blue,", "Sugar is 
sweet,",
              "And so are you."];
    auto d2 = differ(a, b);
}

Here's the error output:

Excluding package.d file from test due to 
https://issues.dlang.org/show_bug.cgi?id=11847
src/package.d(50,35): Error: no property opCall for type 
diffrange.Diff!(dchar[]), did you mean new Diff!(dchar[])?
src/package.d(57,21): Error: template instance 
diffrange.differ!(dchar[]) error instantiating
src/package.d(50,35): Error: no property opCall for type 
diffrange.Diff!(string[]), did you mean new Diff!(string[])?
src/package.d(62,21): Error: template instance 
diffrange.differ!(string[]) error instantiating
/home/mark/opt/bin/ldc2 failed with exit code 1.

Curiously the "Excluding package.d file" message isn't true.
Not that I mind, I just want to be able to get it going.

Thanks!

PS The line numbers don't match because I've deleted some structs 
& enums that are above the class but which aren't used yet.