Wow, I never noticed that this file exists, even though I'm
routinely doing metaprogramming-heavy stuff…
The obligatory references to other »meta« code:
[1]
https://github.com/sinfu/phobos-sandbox/blob/master/std/internal/meta/meta.d
[2]
https://github.com/PhilippeSigaud/dranges/blob/master/templates.d
[3] https://gist.github.com/1191885 (this was a one-off selection
of my own code for a NG discussion, feel free to ask for more/an
updated version)
On Friday, 22 June 2012 at 18:10:38 UTC, Alex Rønne Petersen
wrote:
On 22-06-2012 20:06, Jonathan M Davis wrote:
The question is what to do with it. […]
Yeah, I really think we need to ship this stuff.
I'd rather not ship it without more consideration and a proper
review process. Yes, functionality supporting more advanced
manipulation of compile time lists/tuples is definitely needed
(cf. the countless std.meta discussions), but:
1) The term TypeList does not make much sense at all, as compile
time tuples/lists aren't at all restricted to types. In
retrospect, giving TypeTuples the name they bear was a mistake,
we shouldn't repeat it. The genreral template would become
meta.List (no, I never got around to finishing my proposal), and
a meta.TypeList constructor could then ensure that only types are
accepted (similar for meta.ExprList, or whatever good names there
might be).
2) I'm not sure if introducing a concept which is essentially
the same as TypeTuple, but doesn't entirely replace it for
»high-level« use, is the right way to go. Yes, functional-style
head/tail lists are a natural fit for many meta-algorithms, since
the semantics of templates mostly require a very functional style
anyway (you'll frequently see T[0] and T[1 .. $] in type
tuples-heavy code). And yes, the »auto-expanding« property of
TypeTuples can sometimes lead to unexpected results, and for some
»higher-order« templates, you need to confine them into another
template (imagine TypeList with only toTuple, see [1], [3]).
But on the other hand, type tuples are deeply anchored in the
language (variadics, ...), and better match the general
array/range theme of D, thus probably reducing »mental
overhead« for people not used to functional programming.
This is not to say that I don't find the std.typelist concept
interesting. We just should be very clear on how to go forward
with »meta« algorithms in Phobos before going forward with
this. Developing std.typelist and std.typetuple side by side, and
then at some later point also introducing std.meta doesn't make
much sense, in my eyes.
3) I think going for an explicit »apply« method for template
predicate is not worth the hassle in the general case (in user
code, which usually just passes predicates to higher order
templates). Instead, I prefer using an explicit Apply/Instantiate
template where needed to overcome grammar limitations, which
mostly is in the implementation of the primitives.
4) I know this is picking nits at this stage, but I think And/Or
should implement short-cut evaluation, like in [3].
One more thing I noticed is that the module doc comment solely
lists Bartosz as the author, but the file has another copyright
comment mentioning Burton Radons. This situation should
definitely be clarified.
David
