From 76e2a78cdc90b17cd6d75df4ec9ec6f54a4532fd Mon Sep 17 00:00:00 2001 From: Nicola 'tekNico' Larosa Date: Sun, 21 Jan 2018 19:29:35 +0100 Subject: [PATCH] part10.rs lines shortened --- src/part10.rs | 144 +++++++++++++++++++++++++++++--------------------- 1 file changed, 83 insertions(+), 61 deletions(-) diff --git a/src/part10.rs b/src/part10.rs index 0c90369..a28b611 100644 --- a/src/part10.rs +++ b/src/part10.rs @@ -9,120 +9,137 @@ 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 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 reference. +//@ 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 reference. trait Action { fn do_action(&mut self, digit: u64); } -// Now we can write a function that takes some `a` of a type `A` such that we can call `do_action` on `a`, passing it every digit. +// Now we can write a function that takes some `a` of a type `A` such that we can call `do_action` +// on `a`, passing it every digit. impl BigInt { fn act_v1(&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 reference, so mutation could indeed happen. + //@ 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 reference, so + //@ mutation could indeed happen. for digit in self { a.do_action(digit); /*@*/ } } } -//@ As the next step, we need to come up with some action, and write an appropriate implementation of `Action` for it. -//@ So, let's say we want to print every digit, and to make this less boring, we want the digits to be prefixed by some -//@ arbitrary string. `do_action` has to know this string, so we store it in `self`. +//@ As the next step, we need to come up with some action, and write an appropriate implementation +//@ of `Action` for it. So, let's say we want to print every digit, and to make this less boring, +//@ we want the digits to be prefixed by some arbitrary string. `do_action` has to know this +//@ string, so we store it in `self`. struct PrintWithString { prefix: String, } impl Action for PrintWithString { - // 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. + // 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); /*@*/ } } // Finally, this function takes a `BigInt` and a prefix, and prints the digits with the given prefix. -//@ It does so by creating an instance of `PrintWithString`, giving it the prefix, and then passing that to `act_v1`. -//@ Since `PrintWithString` implements `Action`, Rust now knows what to do. +//@ It does so by creating an instance of `PrintWithString`, giving it the prefix, and then passing +//@ that to `act_v1`. Since `PrintWithString` implements `Action`, Rust now knows what to do. fn print_with_prefix_v1(b: &BigInt, prefix: String) { let my_action = PrintWithString { prefix: prefix }; b.act_v1(my_action); } -// Here's a small main function, demonstrating the code above in action. Remember to edit `main.rs` to run it. +// Here's a small main function, demonstrating the code above in action. Remember to edit `main.rs` +// to run it. pub fn main() { let bignum = BigInt::new(1 << 63) + BigInt::new(1 << 16) + BigInt::new(1 << 63); print_with_prefix_v1(&bignum, "Digit: ".to_string()); } // ## Closures -//@ Now, as it turns out, this pattern of describing some form of an action, that can carry in additional data, is very common. -//@ 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 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`. - -// This defines `act` very similar to above, but now we demand `A` to be the type of a closure that mutates its borrowed environment, -// takes a digit, and returns nothing. +//@ Now, as it turns out, this pattern of describing some form of an action, that can carry in +//@ additional data, is very common. 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 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`. + +// This defines `act` very similar to above, but now we demand `A` to be the type of a closure that +// mutates its borrowed environment, takes a digit, and returns nothing. impl BigInt { fn act(&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`. + // 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); /*@*/ } } } -// Now that we saw how to write a function that operates on closures, let's see how to write a closure. +// Now that we saw how to write a function that operates on closures, let's see how to write a +// closure. pub fn print_with_prefix(b: &BigInt, prefix: String) { - //@ The syntax for closures is `|arg1, arg2, ...| code`. Notice that the closure can reference variables like `prefix` that it did not - //@ take as argument - variables that happen to be present *outside* of the closure. We say that the closure *captures* - //@ variables. Rust will now automatically create a type (like `PrintWithString`) for the environment of the closure - //@ with fields for every captured variable, implement the closure trait for this type such that the action performed - //@ is given by the code of the closure, and finally it will instantiate the environment type here at the definition site - //@ of the closure and fill it appropriately. + //@ The syntax for closures is `|arg1, arg2, ...| code`. Notice that the closure can reference + //@ variables like `prefix` that it did not take as argument - variables that happen to be + //@ present *outside* of the closure. We say that the closure *captures* variables. + //@ Rust will now automatically create a type (like `PrintWithString`) for the environment of + //@ the closure with fields for every captured variable, implement the closure trait for this + //@ type such that the action performed is given by the code of the closure, and finally it + //@ will instantiate the environment type here at the definition site of the closure and fill + //@ it appropriately. b.act(|digit| println!("{}{}", prefix, digit) ); } -// You can change `main` to call this function, and you should notice - nothing, no difference in behavior. -// But we wrote much less boilerplate code! +// You can change `main` to call this function, and you should notice - nothing, no difference in +// behavior. But we wrote much less boilerplate code! -// Remember that we decided to use the `FnMut` trait above? This means our closure could actually mutate its environment. -// For example, we can use that to count the digits as they are printed. +// Remember that we decided to use the `FnMut` trait above? This means our closure could actually +// mutate its environment. 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 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 `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 dead reference. + //@ 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 `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 dead reference. b.act(|digit| { println!("{}: {}", count, digit); count = count +1; } ); println!("There are {} digits", count); } // ## Fun with iterators and closures -//@ If you are familiar with functional languages, you are probably aware that one can have lots of fun with iterators and closures. -//@ Rust provides a whole lot of methods on iterators that allow us to write pretty functional-style list manipulation. +//@ If you are familiar with functional languages, you are probably aware that one can have lots of +//@ fun with iterators and closures. Rust provides a whole lot of methods on iterators that allow +//@ us to write pretty functional-style list manipulation. -// Let's say we want to write a function that increments every entry of a `Vec` by some number, then looks for numbers larger than some threshold, and prints them. +// Let's say we want to write a function that increments every entry of a `Vec` by some number, +// then looks for numbers larger than some threshold, and prints them. fn inc_print_threshold(v: &Vec, offset: i32, threshold: i32) { - //@ `map` takes a closure that is applied to every element of the iterator. `filter` removes elements - //@ from the iterator that do not pass the test given by the closure. - //@ - //@ 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. + //@ `map` takes a closure that is applied to every element of the iterator. `filter` removes + //@ elements from the iterator that do not pass the test given by the closure. 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) { println!("{}", i); } } -// Sometimes it is useful to know both the position of some element in a list, and its value. That's where the `enumerate` function helps. +// Sometimes it is useful to know both the position of some element in a list, and its value. +// That's where the `enumerate` function helps. fn print_enumerated(v: &Vec) { - //@ `enumerate` turns an iterator over `T` into an iterator over `(usize, T)`, where the first element just counts the position in the iterator. - //@ We can do pattern matching right in the loop header to obtain names for both the position, and the value. + //@ `enumerate` turns an iterator over `T` into an iterator over `(usize, T)`, where the first + //@ element just counts the position in the iterator. We can do pattern matching right in the + //@ loop header to obtain names for both the position, and the value. for (i, t) in v.iter().enumerate() { println!("Position {}: {}", i, t); } @@ -130,15 +147,20 @@ fn print_enumerated(v: &Vec) { // 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, divisor: i32) -> Vec { - //@ Here, the return type of `collect` is inferred based on the return type of our function. In general, it can return anything implementing - //@ [`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. + //@ Here, the return type of `collect` is inferred based on the return type of our function. In + //@ general, it can return anything implementing + //@ [`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`](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! +// **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](part09.html) | [raw source](workspace/src/part10.rs) | [next](part11.html) +//@ [index](main.html) | [previous](part09.html) | [raw source](workspace/src/part10.rs) | +//@ [next](part11.html) -- 2.30.2