Understanding isInfinite(Range)

[Andrej Mitrovic]

I'd love to see this used more in Phobos. I don't know if there are any drawbacks, but this looks and works nicely:

import std.stdio : writeln;

void main()
{
    writeln(isInputRange!(N));
}

class N
{
    N test;
    
    bool empty()
    {
        return false;
    }
    
    @property
    void popFront()
    {
    }
    
    @property
    N front()
    {
        return test;
    }
}


template isInputRange(R)
{
    enum bool isInputRange = __traits(compiles,
    {
        R r;             // can define a range object
        if (r.empty) {}  // can test for empty
        r.popFront;          // can invoke next
        auto h = r.front; // can get the front of the range
    });
}

If you uncomment some of those methods in class N, then you get back false, which is what you want. Currently isInputRange is defined like so in Phobos:

template isInputRange(R)
{
    enum bool isInputRange = is(typeof(
    {
        R r;             // can define a range object
        if (r.empty) {}  // can test for empty
        r.popFront;          // can invoke next
        auto h = r.front; // can get the front of the range
    }()));
}

It's getting close to LISP! :)

Andrej Mitrovic Wrote:

> Yeah, that could work:
> 
> template isInputRange(R)
> {
>    enum bool isInputRange = __traits(compiles,
>    {
>        R r;             // can define a range object
>        if (r.empty) {}  // can test for empty
>        r.popFront;          // can invoke next
>        auto h = r.front; // can get the front of the range
>    });
> }
> 
> It does look nice. It would look even nicer if __traits gets renamed to meta.
> 
> Stanislav Blinov Wrote:
> 
> > What about __traits(compiles) ?
>