Merge pull request #38 from bzindovic/part04-variable-type-patch

[rust-101.git] / src / part03.rs

diff --git a/src/part03.rs b/src/part03.rs
index a7cfb1aa3919638f20ee8f607d34649f06f959c6..b81bbc371cd7c3a8c2dc3d16de6c83a58d08a319 100644 (file)
--- a/src/part03.rs
+++ b/src/part03.rs
@@ -1,166 +1,131 @@
-// Rust-101, Part 03: Input, Testing
-// =================================
+// Rust-101, Part 03: Input
+// ========================

 
 

-// In part 00, I promised that we would eventually replace `read_vec` by a function
-// that actually asks the user to enter a bunch of numbers. Unfortunately,
-// I/O is a complicated topic, so the code to do that is not pretty - but well,
-// let's get that behind us.
+//@ In part 00, I promised that we would eventually replace `read_vec` by a function
+//@ that actually asks the user to enter a bunch of numbers. Unfortunately,
+//@ I/O is a complicated topic, so the code to do that is not exactly pretty - but well,
+//@ let's get that behind us.

 
 

-// IO/ is provided by the module `std::io`, so we first import that.
-// We also import the I/O *prelude*, which brings a bunch of commonly used I/O stuff
+// 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::*;
 use std::io;
 
 // directly available.
 use std::io::prelude::*;
 use std::io;
 

-// Let's now go over this function line-by-line.
+//@ Let's now go over this function line-by-line. First, we call the constructor of `Vec`
+//@ to create an empty vector. As mentioned in the previous part, `new` here is just
+//@ a static function with no special treatment. While it is possible to call `new`
+//@ for a particular type (`Vec::<i32>::new()`), the common way to make sure we
+//@ get the right type is to annotate a type at the *variable*. It is this variable
+//@ that we interact with for the rest of the function, so having its type available
+//@ (and visible!) is much more useful. Without knowing the return type of `Vec::new`,
+//@ specifying its type parameter doesn't tell us all that much.

 fn read_vec() -> Vec<i32> {
 fn read_vec() -> Vec<i32> {

-    let mut vec = Vec::new();
-    // The central handle to the standard input is made available by `io::stdin()`.
+    let mut vec: Vec<i32> = Vec::<i32>::new();
+    // The central handle to the standard input is made available by the function `io::stdin`.

     let stdin = io::stdin();
     let stdin = io::stdin();

-    println!("Enter a list of numbers, one per line. End with Ctrl-D.");
-    // We would now like to iterate over standard input line-by-line. We can use a `for` loop
-    // 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](http://doc.rust-lang.org/stable/std/io/struct.Stdin.html) for more
-    // details.) 
+    println!("Enter a list of numbers, one per line. End with Ctrl-D (Linux) or Ctrl-Z (Windows).");
+    //@ We would now like to iterate over standard input line-by-line. We can use a `for` loop
+    //@ 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.)

     for line in stdin.lock().lines() {
     for line in stdin.lock().lines() {

-        // The `line` we have here is not yet of type `String`. The problem with I/O is that it can always
-        // go wrong, so `line` has type `io::Result<String>`. This is a lot like `Option<String>` ("a `String` or
-        // nothing"), but in the case of "nothing", there is additional information about the error.
-        // Again, I recommend to check [the documentation](http://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 halts the program (with an appropriate error message) otherwise. Can you find the documentation
-        // of `Result::unwrap()`?
+        // 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<String>`.
+        //@ The problem with I/O is that it can always go wrong. The type of `line` is a lot like
+        //@ `Option<String>` ("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.
+        //@ 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.

         let line = line.unwrap();
         let line = line.unwrap();

-        // Now that we have our `String`, we want to make it an `i32`. `parse` is a method on `String` that
-        // can convert a string to anything. Try finding it's 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. So I use this opportunity to
-        // introduce the syntax for explicitly giving the type parameter of a generic function: `parse::<i32>` is `parse`
-        // with its generic type set to `i32`.
-        match line.parse::<i32>() {
-        // `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),
-            Err(_) => println!("What did I say about numbers?"),
+        // 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 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::<i32>` is `parse` with its generic type set to `i32`.
+        match line.trim().parse::<i32>() {
+            //@ `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)                                       /*@*/
+            },
+            // We don't care about the particular error, so we ignore it with a `_`.
+            Err(_) => {
+                println!("What did I say about numbers?")           /*@*/
+            },

         }
     }
 
     vec
 }
 
         }
     }
 
     vec
 }
 

-// So much for `read_vec`. If there are any questions left, the documentation of the respective function
-// should be very helpful. I will not always provide the links, as the documentation is quite easy to navigate
-// and you should get used to that.
-// 
-// The rest of the code dosn't change, so we just copy it.
-
-enum SomethingOrNothing<T>  {
-    Something(T),
-    Nothing,
-}
-use self::SomethingOrNothing::{Something,Nothing};
-
-trait Minimum : Copy {
-    fn min(a: Self, b: Self) -> Self;
-}
-
-fn vec_min<T: Minimum>(v: Vec<T>) -> SomethingOrNothing<T> {
-    let mut min = Nothing;
-    for e in v {
-        min = Something(match min {
-            Nothing => e,
-            Something(n) => T::min(n, e)
-        });
-    }
-    min
-}
-
-// `::std::cmp::min` is a way to refer to this function without importing `std`.
-// We could also have done `use std::cmp;` and later called `cmp::min`. Try that!
-impl Minimum for i32 {
-    fn min(a: Self, b: Self) -> Self {
-        ::std::cmp::min(a, b)
-    }
-}
+//@ 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.

 
 

-impl SomethingOrNothing<i32> {
-    fn print(self) {
-        match self {
-            Nothing => println!("The number is: <nothing>"),
-            Something(n) => println!("The number is: {}", n),
-        };
-    }
-}
+// 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
+//@ items declared public can be imported elsewhere.
+use part02::{SomethingOrNothing,Something,Nothing,vec_min};

 
 // If you update your `main.rs` to use part 03, `cargo run` should now ask you for some numbers,
 // and tell you the minimum. Neat, isn't it?
 pub fn main() {
     let vec = read_vec();
 
 // If you update your `main.rs` to use part 03, `cargo run` should now ask you for some numbers,
 // and tell you the minimum. Neat, isn't it?
 pub fn main() {
     let vec = read_vec();

-    let min = vec_min(vec);
-    min.print();
+    let min = vec_min(vec);                                         /*@*/
+    min.print();                                                    /*@*/

 }
 
 }
 

-// After all this nit-picking about I/O details, let me show you quickly something unrelated,
-// but really nice: Rust's built-in support for testing.
-// Now that the user can run our program on loads of inputs, we better make sure that it is correct.
-// To be able to test the result of `vec_min`, we first have to write a function that
-// is able to test equality if `SimethingOrNothing`. So let's quickly do that.
-
-// `equals` performs pattern-matching on both `self` and `other` to test the two for being
-// equal. Because we are lazy, we want to write only one `match`. so we group the two into a
-// pair such that we can match on both of them at once. You can read the first arm of the match
-// as testing whether `(self, other)` is `(Nothing, Nothing)`, which is the case exactly if
-// both `self` and `other` are `Nothing`. Similar so for the second arm.
-impl SomethingOrNothing<i32> {
-    fn equals(self, other: Self) -> bool {
-        match (self, other) {
-            (Nothing     , Nothing     ) => true,
-            (Something(n), Something(m)) => n == m,
-            // `_` is the syntax for "I don't care", so this is how you add a default case to your `match`.
-            _ => false,
-        }
+// **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<T>`".
+// 
+// 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.
+pub trait Print {
+    /* Add things here */
+}
+impl Print for i32 {
+    /* Add things here */
+}
+impl<T: Print> SomethingOrNothing<T> {
+    fn print2(self) {
+        unimplemented!()

     }
 }
 
     }
 }
 

-// Now we are almost done! Writing a test in Rust is shockingly simple. Just write a function
-// that takes no arguments as returns nothing, and add `#[test]` right in front of it.
-// That's called an *attribute*, and the `test` attribute, well, declares the function to
-// be a test.
-
-// Within the function, we can then use `panic!` to indicate test failure. Helpfully, there's
-// a macro `assert!` that panics if its argument becomes `false`.
-// Using `assert!` and our brand-new `equals`, we can now call `vec_min` with some lists
-// and make sure it returns The Right Thing.
-#[test]
-fn test_vec_min() {
-    assert!(vec_min(vec![6,325,33,532,5,7]).equals(Something(5)));
-    assert!(vec_min(vec![6,325,33,532]).equals(Something(6)));
-}
-// To execute the test, run `cargo test`. It should tell you that everything is all right.
-// Now that was simple, wasn't it?
-// 
-// **Exercise**: Add a case to `test_vec_min` that checks the behavior on empty lists.
-// 
-// **Exercise**: Change `vec_min` such that everything still compiles, but the test fails.
-// 
-// **Bonus Exercise**: Because `String::parse` is itself generic, you can change `read_vec` to
-// be a generic function that works for any type, not just for `i32`. However, you will have to add
-// a trait bound to `read_vec`, as not every type supports being parsed. <br/>
-// Once you made `vec_min` generic, copy your generic `print` from the previous part. Implement all
-// our traits (`Minimum` and `Print`) for `f32` (32-bit floating-point numbers), and change `part_main()`
-// such that your program now computes the minimum of a list of floating-point numbers. <br/>
-// *Hint*: You can figure out the trait bound `read_vec` needs from the documentation of `String::parse`.
-// Furthermore, `std::cmp::min` works not just for `i32`, but also for `f32`.
+// **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) | next
+//@ [index](main.html) | [previous](part02.html) | [raw source](workspace/src/part03.rs) |
+//@ [next](part04.html)