On 6/16/14 7:32 AM, Sebastian Gesemann wrote:
I think one aspect of the issue is Rust's Trait system itself. It
tries to kill two birds with one stone: (1) Having "Interfaces" with
runtime dispatching where Traits are used as dynamically-sized types
and (2) as type bound for generics. Initially, I found this to be a
very cool Rust feature. But now, I'm not so sure about that anymore.
Back in 2009 when "concepts" were considered for C++ standardization,
I spent much time on understanding the intricacies of that C++ topic.
This initial "concepts" design also tried to define some type
requirements in terms of function signatures. But this actually
interacted somewhat badly with rvalue references (among other things)
and I think this is one of the reasons why "concepts lite" (a new and
simplified incarnation of the concepts design, expected to augment
C++1y standard in form of a technical report) replaced the function
signatures with "usage patterns". As a user of some well-behaved type,
I don't really care about what kind of optimizations it offers for +
or * and how they work. I'm just glad that I can "use" the "pattern"
x*y where x and y refer to instances of some type. Whether the
implementer of that type distinguishes between lvalues and rvalues via
overloading or not is kind of an implementation detail that does not
affect how the type is being used syntactically. So, I expect "C++
concepts lite" to be able to specify type requirements in terms of
"usage patters" in a way that it allows "models" of these "concepts"
to satisfy the requirements in a number of ways (with move
optimizations being optional but possible).
Another thing I'm not 100% comfortable with (yet?) is the funky way
references are used in Rust in combination with auto-borrowing (for
operators and self at least) and auto-deref'ing while at the same
time, they are used as "values" (like C++ pointers as opposed to C++
references). I've trouble putting this into words. But it feels to me
like the lines are blurred which could cause some trouble or bad
surprizes.
I don't really want to debate the entire Rust generics system here.
Patrick
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