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73 lines
1.8 KiB
73 lines
1.8 KiB
% Higher-Rank Trait Bounds (HRTBs)
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Rust's Fn traits are a little bit magic. For instance, we can write the
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following code:
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```rust
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struct Closure<F> {
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data: (u8, u16),
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func: F,
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}
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impl<F> Closure<F>
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where F: Fn(&(u8, u16)) -> &u8,
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{
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fn call(&self) -> &u8 {
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(self.func)(&self.data)
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}
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}
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fn do_it(data: &(u8, u16)) -> &u8 { &data.0 }
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fn main() {
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let clo = Closure { data: (0, 1), func: do_it };
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println!("{}", clo.call());
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}
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```
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If we try to naively desugar this code in the same way that we did in the
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lifetimes section, we run into some trouble:
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```rust,ignore
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struct Closure<F> {
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data: (u8, u16),
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func: F,
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}
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impl<F> Closure<F>
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// where F: Fn(&'??? (u8, u16)) -> &'??? u8,
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{
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fn call<'a>(&'a self) -> &'a u8 {
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(self.func)(&self.data)
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}
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}
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fn do_it<'b>(data: &'b (u8, u16)) -> &'b u8 { &'b data.0 }
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fn main() {
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'x: {
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let clo = Closure { data: (0, 1), func: do_it };
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println!("{}", clo.call());
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}
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}
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```
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How on earth are we supposed to express the lifetimes on F's trait bound? We need
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to provide some lifetime there, but the lifetime we care about can't be named until
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we enter the body of `call`! Also, that isn't some fixed lifetime; call works with
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*any* lifetime `&self` happens to have at that point.
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This job requires The Magic of Higher-Rank Trait Bounds. The way we desugar
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this is as follows:
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```rust,ignore
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where for<'a> F: Fn(&'a (u8, u16)) -> &'a u8,
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```
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(Where `Fn(a, b, c) -> d` is itself just sugar for the unstable *real* Fn trait)
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`for<'a>` can be read as "for all choices of `'a`", and basically produces an
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*inifinite list* of trait bounds that F must satisfy. Intense. There aren't many
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places outside of the Fn traits where we encounter HRTBs, and even for those we
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have a nice magic sugar for the common cases.
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