On 2014-05-17, at 3:54, Kevin Ballard <ke...@sb.org> wrote:
> On May 15, 2014, at 9:54 PM, Daniel Micay <danielmi...@gmail.com> wrote:
>
>> On 16/05/14 12:48 AM, Tommi wrote:
>>> On 2014-05-16, at 7:35, Steven Fackler <sfack...@gmail.com> wrote:
>>>
>>>> Type annotations are not there for the compiler; they're there for people
>>>> reading the code. If I want to use some function I don't want to be forced
>>>> to read the entire implementation to figure out what the lifetime of the
>>>> return value is.
>>>
>>> Just to be sure... I take it you are in fact implying that "yes, it is
>>> possible for the compiler to figure out those lifetimes without the
>>> programmer explicitly specifying them in the function signature"?
>>
>> Lifetimes are based on subtyping, and AFAIK that presents huge problems
>> for type inference. You would need to ask Niko how feasible it would be
>> to infer it globally. I used to run into all sorts of problems with the
>> *local* inference but it got a lot better.
>
> I don't think global inference even is possible for this, because the
> lifetimes affect borrowing. Example:
>
> pub trait Foo {
> fn foo(&self) -> &str;
> }
>
> This is a trait that defines a method `foo()` that returns a reference. I've
> elided the lifetimes. Now tell me, if I'm calling this method through a
> generic (or a trait object), what is the lifetime of the returned value? Is
> it the same as `&self`, meaning does it need to borrow the receiver? Or is it
> 'static (which won't borrow the receiver)?
>
> -Kevin
I don't think generics (static polymorphism) poses a problem, because for those
the compiler knows exactly which function a call to 'a.foo()' resolves to. But
your point about trait objects (dynamic polymorphism) is good. For trait
objects the compiler doesn't know which function is being called. So, it seems
that explicit named lifetime notation is necessary at least for functions in
traits.
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