avoid using std::cmp::min, for it doesn't support f32...

[rust-101.git] / src / part02.rs

diff --git a/src/part02.rs b/src/part02.rs
index d0cbbf77d93dae99d0e3c71e0a1d92bf1a17662d..e4f50be9d4694bb518d64c9fab65752ccaf7ffb5 100644 (file)
--- a/src/part02.rs
+++ b/src/part02.rs
@@ -1,12 +1,11 @@
 // Rust-101, Part 02: Generic types, Traits
 // ========================================
 
-use std;
+// Let us for a moment reconsider the type `NumberOrNothing`. Isn't it a bit annoying that we
+// had to hard-code the type `i32` in there? What if tomorrow, we want a `CharOrNothing`, and
+// later a `FloatOrNothing`? Certainly we don't want to re-write the type and all its inherent methods.
 
-// Let us for a moment reconsider the type `NumberOrNothing`. Isn't it a bit
-// annoying that we had to hard-code the type `i32` in there? What if tomorrow,
-// we want a `CharOrNothing`, and later a `FloatOrNothing`? Certainly we don't
-// want to re-write the type and all its inherent methods.
+// ## Generic datatypes
 
 // The solution to this is called *generics* or *polymorphism* (the latter is Greek,
 // meaning "many shapes"). You may know something similar from C++ (where it's called
@@ -22,16 +21,16 @@ pub use self::SomethingOrNothing::*;
 // `SomethingOrNothing<i32>` to get back our `NumberOrNothing`.
 type NumberOrNothing = SomethingOrNothing<i32>;
 // However, we can also write `SomethingOrNothing<bool>` or even `SomethingOrNothing<SomethingOrNothing<i32>>`.
-// In fact, such a type is so useful that it is already present in the standard
-// library: It's called an *option type*, written `Option<T>`.
-// Go check out its [documentation](http://doc.rust-lang.org/stable/std/option/index.html)!
+// In fact, such a type is so useful that it is already present in the standard library: It's called an
+// *option type*, written `Option<T>`. Go check out its [documentation](http://doc.rust-lang.org/stable/std/option/index.html)!
 // (And don't worry, there's indeed lots of material mentioned there that we did not cover yet.)
 
+// ## Generic `impl`, Static functions
 // The types are so similar, that we can provide a generic function to construct a `SomethingOrNothing<T>`
 // from an `Option<T>`, and vice versa.
-
 // **Exercise 02.1**: Implement such functions! I provided a skeleton of the solution. Here,
-// `panic!` is another macro. This one terminates execution with the given message.
+// `unimplemented!` is another macro. This one terminates execution saying that something has not yet
+// been implemented.
 // 
 // Notice the syntax for giving generic implementations to generic types: Think of the first `<T>` 
 // as *declaring* a type variable ("I am doing something for all types `T`"), and the second `<T>` as
@@ -42,11 +41,11 @@ type NumberOrNothing = SomethingOrNothing<i32>;
 // Remember that `self` is the `this` of Rust, and implicitly has type `Self`.
 impl<T> SomethingOrNothing<T> {
     fn new(o: Option<T>) -> Self {
-        panic!("Not yet implemented.")
+        unimplemented!()
     }
 
     fn to_option(self) -> Option<T> {
-        panic!("Not yet implemented.")
+        unimplemented!()
     }
 }
 // Observe how `new` does *not* have a `self` parameter. This corresponds to a `static` method
@@ -58,6 +57,7 @@ fn call_constructor(x: i32) -> SomethingOrNothing<i32> {
     SomethingOrNothing::new(Some(x))
 }
 
+// ## Traits
 // Now that we have a generic `SomethingOrNothing`, wouldn't it be nice to also gave a generic
 // `vec_min`? Of course, we can't take the minimum of a vector of *any* type. It has to be a type
 // supporting a `min` operation. Rust calls such properties that we may demand of types *traits*.
@@ -108,18 +108,18 @@ pub fn vec_min<T: Minimum>(v: Vec<T>) -> SomethingOrNothing<T> {
 // This behavior is similar to C++ templates. The optimizer (Rust is using LLVM) then has all the
 // information it could want to, e.g., inline function calls.
 
+// ## Trait implementations
 // To make the function usable with a `Vec<i32>`, we implement the `Minimum` trait for `i32`.
 impl Minimum for i32 {
     fn min(self, b: Self) -> Self {
-        std::cmp::min(self, b)
+        if self < b { self } else { b }
     }
 }
 
-// In order to run our code and see the result, we again provide a `print` function.
-// This also shows that we can have multiple `impl` blocks for the same type (remember
-// that `NumberOrNothing` is just a type alias for `SomethingOrNothing<i32>`), and we
-// can provide some methods only for certain instances of a generic type.
-impl NumberOrNothing{
+// We again provide a `print` function. This also shows that we can have multiple `impl` blocks
+// for the same type (remember that `NumberOrNothing` is just a type alias for `SomethingOrNothing<i32>`),
+// and we can provide some methods only for certain instances of a generic type.
+impl NumberOrNothing {
     pub fn print(self) {
         match self {
             Nothing => println!("The number is: <nothing>"),
@@ -142,4 +142,7 @@ pub fn main() {
 
 // If this printed `3`, then you generic `vec_min` is working! So get ready for the next part.
 
+// **Exercise 02.2**: Change your program such that it computes the minimum of a `Vec<f32>` (where `f32` is the type
+// of 32-bit floating-point numbers). You should not change `vec_min` in any way, obviously!
+
 // [index](main.html) | [previous](part01.html) | [next](part03.html)