tuning

[rust-101.git] / src / part07.rs

diff --git a/src/part07.rs b/src/part07.rs
index 92da555374ed969f64f10531978685de1a3930ab..65449951eaf37a9fd17b188395296577545fa6a1 100644 (file)
--- a/src/part07.rs
+++ b/src/part07.rs
@@ -1,54 +1,150 @@
-use part05::BigInt;
+// Rust-101, Part 07: Operator Overloading, Tests, Formatting
+// ==========================================================

 
 

+pub use part05::BigInt;
+
+// With our new knowledge of lifetimes, we are now able to write down the desired type of `min`:
+//@ We want the function to take two borrows *of the same lifetime*, and then
+//@ return a borrow of that lifetime. If the two input lifetimes would be different, we
+//@ would not know which lifetime to use for the result.

 pub trait Minimum {
 pub trait Minimum {

-    /// Return the smaller of the two

     fn min<'a>(&'a self, other: &'a Self) -> &'a Self;
 }
 
     fn min<'a>(&'a self, other: &'a Self) -> &'a Self;
 }
 

-/// Return a pointer to the minimal value of `v`.
+//@ Now we can implement a generic function `vec_min` that works on above trait.
+//@ The code is pretty much straight-forward, and Rust checks that all the
+//@ lifetimes actually work out. Observe that we don't have to make any copies!

 pub fn vec_min<T: Minimum>(v: &Vec<T>) -> Option<&T> {
 pub fn vec_min<T: Minimum>(v: &Vec<T>) -> Option<&T> {

-    let mut min = None;
+    let mut min: Option<&T> = None;

     for e in v {
     for e in v {

-        min = Some(match min {
-            None => e,
-            Some(n) => e.min(n)
-        });
+        min = Some(match min {                                      /*@*/
+            None => e,                                              /*@*/
+            Some(n) => n.min(e)                                     /*@*/
+        });                                                         /*@*/

     }
     min
 }
     }
     min
 }

-// Notice that `Option<&T>` is technically (leaving the borrowing story aside) a pointer to a `T`,
-// that could optionally be invalid. In other words, it's just like a pointer in C(++) or Java
-// that can be `NULL`! However, thanks to `Option` being an `enum`, we cannot forget to check
-// the pointer for validity, avoiding the safety issues of C(++). At the same time, when we
-// have a borrow like `v` above that's not an `Option`, we *know* that is has to be a valid
-// pointer, so we don't even need to do a `NULL`-check.<br/>
-// Also, if you are worried about wasting space, notice that Rust knows that `&T` can never be
-// `NULL`, and hence optimizes `Option<&T>` to be no larger than `&T`. The `None` case is represented
-// as `NULL`. This is another great example of a zero-cost abstraction: `Option<&T>` is exactly like
-// a pointer in C(++), if you look at what happens during execution - but it's much safer to use.
+//@ Notice that the return type `Option<&T>` is technically (leaving the borrowing story aside) a
+//@ pointer to a `T`, that could optionally be invalid. In other words, it's just like a pointer in
+//@ C(++) or Java that can be `NULL`! However, thanks to `Option` being an `enum`, we cannot forget
+//@ to check the pointer for validity, avoiding the safety issues of C(++).<br/>
+//@ Also, if you are worried about wasting space, notice that Rust knows that `&T` can never be
+//@ `NULL`, and hence optimizes `Option<&T>` to be no larger than `&T`. The `None` case is represented
+//@ as `NULL`. This is another great example of a zero-cost abstraction: `Option<&T>` is exactly like
+//@ a pointer in C(++), if you look at what happens during execution - but it's much safer to use.

 
 

+// **Exercise 07.1**: For our `vec_min` to be usable with `BigInt`, you will have to provide an implementation of
+// `Minimum`. You should be able to pretty much copy the code you wrote for exercise 06.1. You should *not*
+// make any copies!

 impl Minimum for BigInt {
     fn min<'a>(&'a self, other: &'a Self) -> &'a Self {
 impl Minimum for BigInt {
     fn min<'a>(&'a self, other: &'a Self) -> &'a Self {

+        unimplemented!()
+    }
+}
+
+// ## Operator Overloading
+//@ How can we know that our `min` function actually does what we want it to do? One possibility
+//@ here is to do *testing*. Rust comes with nice built-in support for both unit tests and integration
+//@ tests. However, before we go there, we need to have a way of checking whether the results of function calls are
+//@ correct. In other words, we need to define how to test equality of `BigInt`. Being able to
+//@ test equality is a property of a type, that - you guessed it - Rust expresses as a trait: `PartialEq`.
+
+//@ Doing this for `BigInt` is fairly easy, thanks to our requirement that there be no trailing zeros. We simply
+//@ re-use the equality test on vectors, which compares all the elements individually.
+//@ The `inline` attribute tells Rust that we will typically want this function to be inlined.
+impl PartialEq for BigInt {
+    #[inline]
+    fn eq(&self, other: &BigInt) -> bool {

         debug_assert!(self.test_invariant() && other.test_invariant());
         debug_assert!(self.test_invariant() && other.test_invariant());

-        if self.data.len() < other.data.len() {
-            self
-        } else if self.data.len() > other.data.len() {
-            other
-        } else {
-            // compare back-to-front, i.e., most significant digit first
-            let mut idx = self.data.len()-1;
-            while idx > 0 {
-                if self.data[idx] < other.data[idx] {
-                    return self;
-                } else if self.data[idx] > other.data[idx] {
-                    return other;
-                }
-                else {
-                    idx = idx-1;
-                }
-            }
-            // the two are equal
-            return self;
-        }
+        self.data == other.data                                     /*@*/

     }
 }
     }
 }

+
+//@ Since implementing `PartialEq` is a fairly mechanical business, you can let Rust automate this
+//@ by adding the attribute `derive(PartialEq)` to the type definition. In case you wonder about
+//@ the "partial", I suggest you check out the documentation of [`PartialEq`](http://doc.rust-lang.org/std/cmp/trait.PartialEq.html)
+//@ and [`Eq`](http://doc.rust-lang.org/std/cmp/trait.Eq.html). `Eq` can be automatically derived as well.
+
+// Now we can compare `BigInt`s. Rust treats `PratialEq` special in that it is wired to the operator `==`:
+//@ That operator can not be used on our numbers! Speaking in C++ terms, we just overloaded the `==` operator
+//@ for `BigInt`. Rust does not have function overloading (i.e., it will not dispatch to different
+//@ functions depending on the type of the argument). Instead, one typically finds (or defines) a
+//@ trait that catches the core characteristic common to all the overloads, and writes a single
+//@ function that's generic in the trait. For example, instead of overloading a function for all
+//@ the ways a string can be represented, one writes a generic functions over [ToString](http://doc.rust-lang.org/std/string/trait.ToString.html).
+//@ Usually, there is a trait like this that fits the purpose - and if there is, this has the great
+//@ advantage that any type *you* write, that can convert to a string, just has to implement
+//@ that trait to be immediately usable with all the functions out there that generalize over `ToString`.
+//@ Compare that to C++ or Java, where the only chance to add a new overloading variant is to
+//@ edit the class of the receiver.
+//@ 
+//@ Why can we also use `!=`, even though we just overloaded `==`? The answer lies in what's called a *default implementation*.
+//@ If you check out the documentation of `PartialEq` I linked above, you will see that the trait actually provides
+//@ two methods: `eq` to test equality, and `ne` to test inequality. As you may have guessed, `!=` is wired to `ne`.
+//@ The trait *definition* also provides a default implementation of `ne` to be the negation of `eq`. Hence you can just
+//@ provide `eq`, and `!=` will work fine. Or, if you have a more efficient way of deciding inequality, you can provide
+//@ `ne` for your type yourself.
+fn compare_big_ints() {
+    let b1 = BigInt::new(13);
+    let b2 = BigInt::new(37);
+    println!("b1 == b1: {} ; b1 == b2: {}; b1 != b2: {}", b1 == b1, b1 == b2, b1 != b2);
+}
+
+// ## Testing
+// With our equality test written, we are now ready to write our first testcase.
+//@ It doesn't get much simpler: You just write a function (with no arguments or return value), and give it
+// the `test` attribute. `assert!` is like `debug_assert!`, but does not get compiled away in a release build.
+#[test]
+fn test_min() {
+    let b1 = BigInt::new(1);
+    let b2 = BigInt::new(42);
+    let b3 = BigInt::from_vec(vec![0, 1]);
+
+    assert!(*b1.min(&b2) == b1);                                    /*@*/
+    assert!(*b3.min(&b2) == b2);                                    /*@*/
+}
+// Now run `cargo test` to execute the test. If you implemented `min` correctly, it should all work!
+
+// ## Formatting
+//@ There is also a macro `assert_eq!` that's specialized to test for equality, and that prints the two
+//@ values (left and right) if they differ. To be able to do that, the macro needs to know how to format
+//@ the value for printing. This means that we - guess what? - have to implement an appropriate trait.
+//@ Rust knows about two ways of formatting a value: `Display` is for pretty-printing something in a way
+//@ that users can understand, while `Debug` is meant to show the internal state of data and targeted at
+//@ the programmer. The latter is what we want for `assert_eq!`, so let's get started.
+
+// All formating is handled by [`std::fmt`](http://doc.rust-lang.org/std/fmt/index.html). I won't explain
+// all the details, and refer you to the documentation instead.
+use std::fmt;
+
+//@ In the case of `BigInt`, we'd like to just output our internal `data` array, so we
+//@ simply call the formating function of `Vec<u64>`.
+impl fmt::Debug for BigInt {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.data.fmt(f)
+    }
+}
+//@ `Debug` implementations can be automatically generated using the `derive(Debug)` attribute.
+
+// Now we are ready to use `assert_eq!` to test `vec_min`.
+/*#[test]*/
+fn test_vec_min() {
+    let b1 = BigInt::new(1);
+    let b2 = BigInt::new(42);
+    let b3 = BigInt::from_vec(vec![0, 1]);
+
+    let v1 = vec![b2.clone(), b1.clone(), b3.clone()];
+    let v2 = vec![b2.clone(), b3.clone()];
+    assert_eq!(vec_min(&v1), Some(&b1));                            /*@*/
+    assert_eq!(vec_min(&v2), Some(&b2));                            /*@*/
+}
+
+// **Exercise 07.1**: Add some more testcases. In particular, make sure you test the behavior of
+// `vec_min` on an empty vector. Also add tests for `BigInt::from_vec` (in particular, removing
+// trailing zeros). Finally, break one of your functions in a subtle way and watch the test fail.
+// 
+// **Exercise 07.2**: Go back to your good ol' `SomethingOrNothing`, and implement `Display` for it. (This will,
+// of course, need a `Display` bound on `T`.) Then you should be able to use them with `println!` just like you do
+// with numbers, and get rid of the inherent functions to print `SomethingOrNothing<i32>` and `SomethingOrNothing<f32>`.
+
+//@ [index](main.html) | [previous](part06.html) | [next](main.html)