On 05.12.2017 20:11, H. S. Teoh wrote:
On Tue, Dec 05, 2017 at 07:09:50PM +0000, Adam D. Ruppe via Digitalmars-d wrote:
On Tuesday, 5 December 2017 at 19:01:48 UTC, A Guy With a Question wrote:
alias Items(T) = Array!Item(T);
try:
Array!(Item!(T))
I'm not quite sure I understand how to create a generic container
interface or class in D.
Just using the parenthesis should help. The thing with A!B!C is that
the reader can easily be confused: did you mean A!(B!C) or (A!B)!C or
what? Now the compiler is a bit stupider about this than it should be
IMO - it should be able to figure it out and the meaning be fairly
clear with association - but it isn't, so you can and must write
parens to clarify most the time.
Here's an idea for a DIP: make '!' right-to-left associative (i.e.,
similar to the ^^ exponentiation operator), so that A!B!C is understood
as A!(B!C).
Rationale: the most common use cases are of the A!(B!C) variety; it's
pretty rare IME to need the (A!B)!C form, since usually a template
expands to a type, which can then be passed to another template, i.e.,
A!(B!C). The (A!B)!C form is when the template instantiated with B
produces another template that takes another type argument. There
aren't many use cases for this that I can think of.
...
Curried templates are actually common enough in Phobos. (map, filter, etc.)
Though the point is probably moot, because in the current grammar you'd
need parentheses as soon as the template argument is anything more than
a single token, i.e., you can write A!int but you have to write
A!(const(int)). And in the cases where you actually want something of
the form A!(B!C), usually the arguments are themselves pretty
complicated, so wouldn't benefit from top-level associativity anyway.
T
IMHO the inconsistency with function call syntax ought to kill this
proposal. Also, it is a bit more confusing than it is useful.
Related: It's quite annoying that (A!B)!C does not work because the
parser thinks it's a C-style cast and errors out even though C-style
casts are not even valid D syntax.
