Andrei Alexandrescu wrote:
Don wrote:
Jeremie Pelletier wrote:
http://bartoszmilewski.wordpress.com/2009/10/21/what-does-haskell-have-to-do-with-c/
Bartosz's second part of 'Template Metaprogramming Made Easy (Huh?)',
its quite a read :)
Yes, it is excellent. Two comments:
(1) Bartosz's D examples make me seriously question 'static foreach'
which is scheduled for implementation (bugzilla 3377).
If implemented, it will be a source of frustration, since it will not
be very usable inside templates. The ability to exit from a 'static
foreach' is something which is possible with a 'return'-style syntax,
but is not possible with the 'eponymous template hack'.
I think breaking early out of a static foreach is not necessary (but
indeed convenient) for writing good loops.
(2) It seems pretty clear that we need to allow the eponymous trick to
continue to work when more than one template member is present. I
think everyone who's ever attempted template metaprogramming in D has
proposed it!
Yes, that was on the list for a long time. Bartosz even has participated
to many related discussions. I'm surprised the article made it seem an
unescapable matter of principles, when it clearly is a trivially fixable
bug in the language definition.
Yes, looking at the compiler source, it doesn't look too difficult. The
fact that something like this works:
template foo(int X)
{
static if (bar!(X)) { const int foo = 57; }
else { const char [] foo = "abc"; }
}
makes it pretty clear that the difficult part has already been done.
I don't know what happens with template mixins, though. I hate template
mixins (and I'm not convinced they're useful for anything, either).
We discussed using "this" instead of the template's name, but that has a
few ambiguity problems. Currently, we want to allow a template to define
private symbols in addition to the eponymous trick (a term that Bartosz
shouldn't have implied paternity of, sigh). Those private members would
be accessible from inside the template's definition, but not from the
outside. That would effectively remove the issue.
Andrei
