我认为以下代码可行:
use std::num::{Num};
use std::fmt::{Show};
pub type GradFn<T : Num> = for<'a> fn(&'a [T]) -> (T, Vec<T>);
fn minimize<T : Show, F>(f : GradFn<T>, x0 : &[T]) {
// some no-op to test types
print!("{}",f(x0))
}
fn main() {
let xSquared : GradFn<f64> = |x : &[f64]| -> (f64, Vec<f64>) {
return (x[0] * x[0], vec![2.0 * x[0]]);
};
let (fx, grad) = xSquared(vec![2.0f64].as_slice());
print!("{}", fx);
}
但我收到编译错误(见here):
<anon>:12:32: 14:4 error: mismatched types: expected `fn(&'a [f64]) -> (f64, collections::vec::Vec<f64>)`, found `|&[f64]| -> (f64, collections::vec::Vec<f64>)` (expected extern fn, found fn)
<anon>:12 let xSquared : GradFn<f64> = |x : &[f64]| -> (f64, Vec<f64>) {
<anon>:13 return (x[0] * x[0], vec![2.0 * x[0]]);
<anon>:14 };
答案 0 :(得分:4)
fn
没有定义闭包类型;它定义了裸函数指针(即指向用fn
关键字定义的函数的指针)。这就是为什么你不能为GradFn
指定一个闭包的原因。相反,您要使用Fn
,FnMut
或FnOnce
。
我需要做一些更改才能编译这段代码:
minimize
上,您编写的f
参数会按值接收未归类的类型,这是禁止的。您还会添加一个您不使用的F
类型参数。您可能想要约束F
并使用F
作为f
的类型。xSquared
上的类型注释,这是不必要的。这让我可以完全删除类型别名。这是最终的代码:
#![feature(unboxed_closures)]
use std::num::{Num};
use std::fmt::{Show};
fn minimize<T: Show, F: FnMut(&[T]) -> (T, Vec<T>)>(mut f: F, x0: &[T]) {
// some no-op to test types
print!("{}", f(x0))
}
fn main() {
let xSquared = |x: &[f64]| -> (f64, Vec<f64>) {
return (x[0] * x[0], vec![2.0 * x[0]]);
};
let (fx, grad) = xSquared(vec![2.0f64].as_slice());
print!("{}", fx);
}
答案 1 :(得分:0)
如果你的GradFn
实际上是一个裸函数指针(而不是闭包),你可以保留类型别名,如下所示:
use std::num::Num;
use std::fmt::Show;
// this type has to point to a bare function, not a closure
pub type GradFn<T> = for<'a> fn(&'a [T]) -> (T, Vec<T>);
fn minimize<T>(f : GradFn<T>, x0 : &[T])
where T: Show + Num {
// some no-op to test types
println!("{}",f(x0))
}
fn main() {
// this is now a bare function
fn x_squared(x : &[f64]) -> (f64, Vec<f64>) {
return (x[0] * x[0], vec![2.0 * x[0]]);
}
// and this is a pointer to it, that uses your type alias
let x_sq : GradFn<f64> = x_squared;
let (fx, grad) = x_sq(&[2f64]);
println!("fx: {} - grad: {}", fx, grad);
minimize(x_sq, &[3f64]);; // works with minimize as well
}