X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/fb470b22751497032d97accad2ef5850d520a589..bc8b6a556edacde1fcdb3bc9c1568373bc31d2d8:/src/part03.rs diff --git a/src/part03.rs b/src/part03.rs index 513148b..b81bbc3 100644 --- a/src/part03.rs +++ b/src/part03.rs @@ -6,7 +6,7 @@ //@ I/O is a complicated topic, so the code to do that is not exactly pretty - but well, //@ let's get that behind us. -// I/O is provided by the module `std::io`, so we first have import that with `use`. +// I/O is provided by the module `std::io`, so we first have to import that with `use`. // We also import the I/O *prelude*, which makes a bunch of commonly used I/O stuff // directly available. use std::io::prelude::*; @@ -29,41 +29,46 @@ fn read_vec() -> Vec { //@ for that, but there is a catch: What happens if there is some other piece of code running //@ concurrently, that also reads from standard input? The result would be a mess. Hence //@ Rust requires us to `lock` standard input if we want to perform large operations on - //@ it. (See [the documentation](https://doc.rust-lang.org/stable/std/io/struct.Stdin.html) for more - //@ details.) + //@ it. (See [the documentation](https://doc.rust-lang.org/stable/std/io/struct.Stdin.html) for + //@ more details.) for line in stdin.lock().lines() { // Rust's type for (dynamic, growable) strings is `String`. However, our variable `line` // here is not yet of that type: It has type `io::Result`. - //@ The problem with I/O is that it can always go wrong. The type of `line` is a lot like `Option` ("a `String` or - //@ nothing"), but in the case of "nothing", there is additional information about the error. - //@ Again, I recommend to check [the documentation](https://doc.rust-lang.org/stable/std/io/type.Result.html). - //@ You will see that `io::Result` is actually just an alias for `Result`, so click on that to obtain + //@ The problem with I/O is that it can always go wrong. The type of `line` is a lot like + //@ `Option` ("a `String` or nothing"), but in the case of "nothing", there is + //@ additional information about the error. Again, I recommend to check + //@ [the documentation](https://doc.rust-lang.org/stable/std/io/type.Result.html). You will + //@ see that `io::Result` is actually just an alias for `Result`, so click on that to obtain //@ the list of all constructors and methods of the type. - //@ We will be lazy here and just assume that nothing goes wrong: `unwrap` returns the `String` if there is one, - //@ and panics the program otherwise. Since a `Result` carries some details about the error that occurred, - //@ there will be a somewhat reasonable error message. Still, you would not want a user to see such - //@ an error, so in a "real" program, we would have to do proper error handling. + //@ We will be lazy here and just assume that nothing goes wrong: `unwrap` returns the + //@ `String` if there is one, and panics the program otherwise. Since a `Result` carries + //@ some details about the error that occurred, there will be a somewhat reasonable error + //@ message. Still, you would not want a user to see such an error, so in a "real" program, + //@ we would have to do proper error handling. //@ Can you find the documentation of `Result::unwrap`? //@ - // I chose the same name (`line`) for the new variable to ensure that I will never, accidentally, - // access the "old" `line` again. + // I chose the same name (`line`) for the new variable to ensure that I will never, + // accidentally, access the "old" `line` again. let line = line.unwrap(); // Now that we have our `String`, we want to make it an `i32`. //@ We first `trim` the `line` to remove leading and trailing whitespace. - //@ `parse` is a method on `String` that can convert a string to anything. Try finding it's documentation! + //@ `parse` is a method on `String` that can convert a string to anything. Try finding its + //@ documentation! - //@ In this case, Rust *could* figure out automatically that we need an `i32` (because of the return type - //@ of the function), but that's a bit too much magic for my taste. We are being more explicit here: - //@ `parse::` is `parse` with its generic type set to `i32`. + //@ In this case, Rust *could* figure out automatically that we need an `i32` (because of + //@ the return type of the function), but that's a bit too much magic for my taste. We are + //@ being more explicit here: `parse::` is `parse` with its generic type set to `i32`. match line.trim().parse::() { - //@ `parse` returns again a `Result`, and this time we use a `match` to handle errors (like, the user entering - //@ something that is not a number). - //@ This is a common pattern in Rust: Operations that could go wrong will return `Option` or `Result`. - //@ The only way to get to the value we are interested in is through pattern matching (and through helper functions - //@ like `unwrap`). If we call a function that returns a `Result`, and throw the return value away, - //@ the compiler will emit a warning. It is hence impossible for us to *forget* handling an error, - //@ or to accidentally use a value that doesn't make any sense because there was an error producing it. + //@ `parse` returns again a `Result`, and this time we use a `match` to handle errors + //@ (like, the user entering something that is not a number). + //@ This is a common pattern in Rust: Operations that could go wrong will return + //@ `Option` or `Result`. The only way to get to the value we are interested in is + //@ through pattern matching (and through helper functions like `unwrap`). If we call + //@ a function that returns a `Result`, and throw the return value away, the compiler + //@ will emit a warning. It is hence impossible for us to *forget* handling an error, + //@ or to accidentally use a value that doesn't make any sense because there was an + //@ error producing it. Ok(num) => { vec.push(num) /*@*/ }, @@ -77,9 +82,9 @@ fn read_vec() -> Vec { vec } -//@ So much for `read_vec`. If there are any questions left, the documentation of the respective function -//@ should be very helpful. Try finding the one for `Vec::push`. I will not always provide the links, -//@ as the documentation is quite easy to navigate and you should get used to that. +//@ So much for `read_vec`. If there are any questions left, the documentation of the respective +//@ function should be very helpful. Try finding the one for `Vec::push`. I will not always provide +//@ the links, as the documentation is quite easy to navigate and you should get used to that. // For the rest of the code, we just re-use part 02 by importing it with `use`. //@ I already sneaked a bunch of `pub` in part 02 to make this possible: Only @@ -94,17 +99,23 @@ pub fn main() { min.print(); /*@*/ } -// **Exercise 03.1**: Define a trait `Print` to write a generic version of `SomethingOrNothing::print`. -// Implement that trait for `i32`, and change the code above to use it. +// **Exercise 03.1**: The goal is to write a generic version of `SomethingOrNothing::print`. +// To this end, define a trait `Print` that provides (simple) generic printing, and implement +// that trait for `i32`. Then define `SomethingOrNothing::print2` to use that trait, and change +// `main` above to use the new generic `print2` function. // I will again provide a skeleton for this solution. It also shows how to attach bounds to generic // implementations (just compare it to the `impl` block from the previous exercise). // You can read this as "For all types `T` satisfying the `Print` trait, I provide an implementation // for `SomethingOrNothing`". // -// Notice that I called the function on `SomethingOrNothing` `print2` to disambiguate from the `print` defined previously. +// Notice that I called the function on `SomethingOrNothing` `print2` to disambiguate from the +// `print` defined previously. // // *Hint*: There is a macro `print!` for printing without appending a newline. -trait Print { +pub trait Print { + /* Add things here */ +} +impl Print for i32 { /* Add things here */ } impl SomethingOrNothing { @@ -113,7 +124,8 @@ impl SomethingOrNothing { } } -// **Exercise 03.2**: Building on exercise 02.2, implement all the things you need on `f32` to make your -// program work with floating-point numbers. +// **Exercise 03.2**: Building on exercise 02.2, implement all the things you need on `f32` to make +// your program work with floating-point numbers. -//@ [index](main.html) | [previous](part02.html) | [raw source](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part03.rs) | [next](part04.html) +//@ [index](main.html) | [previous](part02.html) | [raw source](workspace/src/part03.rs) | +//@ [next](part04.html)