Re: A thought for template alias parameters?

[Joel Anderson]

On 2/3/2010 9:34 PM, Trip Volpe wrote:
I'm working on a set of handy unit testing accessories for my projects. What 
I'd love to achieve is something like this:
    expectEqual( myFoo,  3 );

Which, on failure, would result in something along the lines of this diagnostic:

    somefile.d:37: Test failure!
         Expected: myFoo == 3
         Actual: 100, 3

Using a bit of template magic with alias parameters, I've come close:

void expectEquals(alias A, alias B)
         ( string file = __FILE__, int line = __LINE__ )
{
        if(A != B) {
                writefln( "Error: %s, line %d", file, line );
                writefln( "    expected: %s == % s",  A.stringof, B.stringof );
                writefln( "    actual: %s, %s", A, B );
        }
}

void main() {
         int myFoo = 100;
         expectEquals! ( myFoo, 3 );
}

This will produce the output

Error: test.d, line 33
     expected: myFoo == 3
     actual: 100, 3


Unfortunately, this isn't quite there yet. Alias parameters can only match 
single symbols, so something like this won't compile:

         int myFoo = 100;
         int myArray[5];
         expectEquals! ( myFoo, myArray[0] );

So my question is, would it be a good idea to add the ability in future 
versions for template parameters to alias entire expressions? It would be 
perfect for making templated assertions like this and probably has many other 
metaprogramming applications. As a recent convert from C++, I still miss the 
ability of macro systems (like GoogleTest) to print highly reflective unit test 
assertion messages.

Would there be any drawbacks or difficulties in doing this that I'm completely 
missing?  :-P

You could potentially use a mixin to do this.  The resulting code would 
look something like this.

void main()
{
          int myFoo = 100;
          mixin(expectEquals! ( "myFoo == 3" ));
}