X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/83e8dbfff2a6fdce785af58dbb13f007c4234cf2..a8d4349a7d6be8d9e09b9af29b481e0c6abb54f1:/src/part10.rs

diff --git a/src/part10.rs b/src/part10.rs
index e0481a8..6fa8f1e 100644
--- a/src/part10.rs
+++ b/src/part10.rs
@@ -1,8 +1,7 @@
 // Rust-101, Part 10: Closures
 // ===========================
 
-use std::io::prelude::*;
-use std::{fmt,io};
+use std::fmt;
 use part05::BigInt;
 
 //@ Assume we want to write a function that does *something* on, say, every digit of a `BigInt`.
@@ -10,10 +9,10 @@ use part05::BigInt;
 //@ our function. In Rust, a natural first attempt to express this is to have a trait for it.
 
 // So, let us define a trait that demands that the type provides some method `do_action` on digits.
-//@ This immediately raises the question: How do we pass `self` to that function? Owned, shared borrow,
-//@ or mutable borrow? The typical strategy to answer this question is to use the strongest
+//@ This immediately raises the question: How do we pass `self` to that function? Owned, shared reference,
+//@ or mutable reference? The typical strategy to answer this question is to use the strongest
 //@ type that still works. Certainly, passing `self` in owned form does not work: Then the function
-//@ would consume `self`, and we could not call it again, on the second digit. So let's go with a mutable borrow.
+//@ would consume `self`, and we could not call it again, on the second digit. So let's go with a mutable reference.
 trait Action {
     fn do_action(&mut self, digit: u64);
 }
@@ -22,9 +21,9 @@ trait Action {
 impl BigInt {
     fn act_v1<A: Action>(&self, mut a: A) {
         //@ Remember that the `mut` above is just an annotation to Rust, telling it that we're okay with `a` being mutated.
-        //@ Calling `do_action` on `a` takes a mutable borrow, so mutation could indeed happen.
+        //@ Calling `do_action` on `a` takes a mutable reference, so mutation could indeed happen.
         for digit in self {
-            a.do_action(digit);
+            a.do_action(digit);                                     /*@*/
         }
     }
 }
@@ -37,10 +36,10 @@ struct PrintWithString {
 }
 
 impl Action for PrintWithString {
-    // Here we perform performs the actual printing of the prefix and the digit. We're not making use of our ability to
+    // Here we perform the actual printing of the prefix and the digit. We're not making use of our ability to
     // change `self` here, but we could replace the prefix if we wanted.
     fn do_action(&mut self, digit: u64) {
-        println!("{}{}", self.prefix, digit);
+        println!("{}{}", self.prefix, digit);                       /*@*/
     }
 }
 
@@ -63,7 +62,7 @@ pub fn main() {
 //@ In general, this is called a *closure*. Closures take some arguments and produce a result, and they have an *environment*
 //@ they can use, which corresponds to the type `PrintWithString` (or any other type implementing `Action`). Again we have the
 //@ choice of passing this environment in owned or borrowed form, so there are three traits for closures in Rust: `Fn`-closures
-//@ get a shared borrow, `FnMut`-closures get a mutable borrow, and `FnOnce`-closures consume their environment (and can hence
+//@ get a shared reference, `FnMut`-closures get a mutable reference, and `FnOnce`-closures consume their environment (and can hence
 //@ be called only once). The syntax for a closure trait which takes arguments of type `T1`, `T2`, ... and returns something
 //@ of type `U` is `Fn(T1, T2, ...) -> U`.
 
@@ -73,7 +72,7 @@ impl BigInt {
     fn act<A: FnMut(u64)>(&self, mut a: A) {
         for digit in self {
             // We can call closures as if they were functions - but really, what's happening here is translated to essentially what we wrote above, in `act_v1`.
-            a(digit);
+            a(digit);                                               /*@*/
         }
     }
 }
@@ -95,11 +94,11 @@ pub fn print_with_prefix(b: &BigInt, prefix: String) {
 // For example, we can use that to count the digits as they are printed.
 pub fn print_and_count(b: &BigInt) {
     let mut count: usize = 0;
-    //@ This time, the environment will contain a field of type `&mut usize`, that will be initialized with a mutable borrow of
+    //@ This time, the environment will contain a field of type `&mut usize`, that will be initialized with a mutable reference of
     //@ `count`. The closure, since it mutably borrows its environment, is able to access this field and mutate `count`
-    //@ through it. Once `act` returns, the closure is destroyed and the borrow of `count` ends. Because closures compile down
+    //@ through it. Once `act` returns, the closure is destroyed and `count` is no longer borrowed. Because closures compile down
     //@ to normal types, all the borrow checking continues to work as usually, and we cannot accidentally leak a closure somewhere
-    //@ that still contains, in its environment, a borrow that has ended.
+    //@ that still contains, in its environment, a dead reference.
     b.act(|digit| { println!("{}: {}", count, digit); count = count +1; } );
     println!("There are {} digits", count);
 }
@@ -115,7 +114,7 @@ fn inc_print_even(v: &Vec<i32>, offset: i32, threshold: i32) {
     //@ 
     //@ Since all these closures compile down to the pattern described above, there is actually no heap allocation going on here. This makes
     //@ closures very efficient, and it makes optimization fairly trivial: The resulting code will look like you hand-rolled the loop in C.
-    for i in v.iter().map(|n| n + offset).filter(|n| *n > threshold) {
+    for i in v.iter().map(|n| *n + offset).filter(|n| *n > threshold) {
         println!("{}", i);
     }
 }
@@ -132,13 +131,14 @@ fn print_enumerated<T: fmt::Display>(v: &Vec<T>) {
 // And as a final example, one can also collect all elements of an iterator, and put them, e.g., in a vector.
 fn filter_vec_by_divisor(v: &Vec<i32>, divisor: i32) -> Vec<i32> {
     //@ Here, the return type of `collect` is inferred based on the return type of our function. In general, it can return anything implementing
-    //@ [`FromIterator`](http://doc.rust-lang.org/stable/std/iter/trait.FromIterator.html).
-    v.iter().filter(|n| *n % divisor == 0).collect()
+    //@ [`FromIterator`](https://doc.rust-lang.org/stable/std/iter/trait.FromIterator.html). Notice that `iter` gives us an iterator over
+    //@ borrowed `i32`, but we want to own them for the result, so we insert a `map` to dereference.
+    v.iter().map(|n| *n).filter(|n| *n % divisor == 0).collect()    /*@*/
 }
 
-// **Exercise 10.1**: Look up the [documentation of `Iterator`](http://doc.rust-lang.org/stable/std/iter/trait.Iterator.html) to learn about more functions
+// **Exercise 10.1**: Look up the [documentation of `Iterator`](https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html) to learn about more functions
 // that can act on iterators. Try using some of them. What about a function that sums the even numbers of an iterator? Or a function that computes the
 // product of those numbers that sit at odd positions? A function that checks whether a vector contains a certain number? Whether all numbers are
 // smaller than some threshold? Be creative!
 
-//@ [index](main.html) | [previous](part08.html) | [next](main.html)
+//@ [index](main.html) | [previous](part09.html) | [raw source](workspace/src/part10.rs) | [next](part11.html)