Understanding isInfinite(Range)

[Philippe Sigaud]

On Mon, Sep 6, 2010 at 23:31, Andrej Mitrovic <andrej.mitrovich at test.com>wrote:

> That still won't work. Observe:



> template isInputRange(R)
> {
>     enum bool isInputRange = isValidCode!(
>     {
>        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
>    });
> }
>
> template isValidCode(alias code) { enum bool isValidCode =
> __traits(compiles, code); }
>
> Instead of returning false, it will give out a compiler error.
>

That's because the part between the curly braces is evaluated before being
passed to the template. And there is no lazy alias.
As Mafi said, you can use a string, it's still the best way to move code
around in D. With q{ ... }, it's palatable.
And no, before you try it, there is no way to pass the {...} to another
template that would stringify it into a q{...} :-)

Maybe, eventually, something like this:

import std.stdio;


template isValidCode(alias code)
{
    template For(T)
    {
        enum bool For = __traits(compiles, code(T.init));
    }
}

void main()
{
   // use an anonymous templated function:
alias isValidCode!((r)
                        {
                            if (r.empty) {}     // can test for empty
                            r.popFront;         // can invoke next
                            auto h = r.front;   // c
                        }
                    ) isInputRange;

//    writeln(isInputRange.For!(int[]));
}

Except DMD doesn't like the commented-out line. Whaoh!



Philippe
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