On 9/29/11 11:00 AM, Graydon Hoare wrote:
You're introducing either require-copy or unsafe-alias problems.
Closure-passing style tells the alias-analysis system what it needs to
keep reference-based iteration safe. You can't touch the vec while
you're pointing into it. Statically. Even if you make copies, it's very
likely nothing you return *from* the iterator can be a reference either
since they'll be hidden state inside the iterator; the alias-analysis
system can't make unsafe code safe.
For some reason the alias analysis doesn't allow me to write vec::eachi
the way I'd like to:
fn eachi<@T>(f: block(T, uint) -> (), v: [mutable? T]) {
let i = 0u;
let l = len(v);
while (i < l) {
f(v[i], i); // fails
i += 1u;
}
}
../src/lib/vec.rs:340:8: 340:9 error: function may alias with argument
0, which is not immutably rooted
../src/lib/vec.rs:340 f(v[i], i);
Instead I have to write:
fn eachi<@T>(f: block(T, uint) -> (), v: [mutable? T]) {
let i = 0u;
let l = len(v);
while (i < l) {
let elem = v[i]; // Satisfy alias checker
f(elem, i);
i += 1u;
}
}
I guess the alias analysis is being defensive against the block here?
Patrick
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