Verbose checking of range category

[Andrej Mitrovic]

Why not just add an extra string parameter to constraint functions
that will show a custom error message? This kind of works (although
you still get standard errors):

import std.conv;
import std.stdio;

alias int cool;

bool isCool(T, string X)()
{
    static if (is(T : cool))
    {
        return true;
    }
    else
    {
        pragma(msg, "Constraint failure: " ~ X);
        return false;
    }
}

void foo(X)(X x) if (isCool!(X, "foo(X) argument must be cool!")())
{
}

void main()
{
    cool myvar;
    double myvar2;
    foo!()(myvar);
    foo!()(myvar2);
}

On 12/11/10, Robert Jacques <sandford at jhu.edu> wrote:
> On Sat, 11 Dec 2010 12:15:31 -0500, Andrei Alexandrescu
> <SeeWebsiteForEmail at erdani.org> wrote:
> [snip]
>> This program will generate a valid executable, but will also print
>> during compilation:
>>
>> Type int is not a random access range because:
>>    no empty property
>>    no front property
>>    no popFront method
>>    no indexing
>>    no slicing
>>
>> When a programmer has an odd issue with a range check, turning
>> verboseness of checks could help.
>>
>> What do you think?
>>
>>
>> Andrei
>
> An issue with this is that failed template type checks are both extremely
> common and expected in template code: i.e. static
> if(isRandomAccessRange!T) {...}. So, you'll get a lot of spurious error
> messages slowly. Add in parallel builds, and the last error message won't
> even be the one you're looking for. So, while I think the concept is
> usefully, I'd view this implementation as an intermediate stepping stone
> at best (and a distraction from you fixing other bugs at worse). I'd
> recommend, as an alternative, to have specific debug versions of the
> checks, i.e. isRandomAccessRangeDebug, as a way to avoid false positives.
>