Give more detail about eq in book about enums

Author: mdinger

src/doc/trpl/compound-data-types.md

@@ -200,8 +200,62 @@ destructuring `let`, as we discussed previously in 'tuples.' In this case, the
 ## Enums
 
 Finally, Rust has a "sum type", an *enum*. Enums are an incredibly useful
-feature of Rust, and are used throughout the standard library. This is an enum
-that is provided by the Rust standard library:
+feature of Rust, and are used throughout the standard library. An `enum` is
+a type which ties a set of alternates to a specific name. For example, below
+we define `Character` to be either a `Digit` or something else. These
+can be used via their fully scoped names: `Character::Other` (more about `::`
+below).
+
+```rust
+enum Character {
+    Digit(i32),
+    Other,
+}
+```
+
+An `enum` variant can be defined as most normal types. Below are some example
+types have been listed which also would be allowed in an `enum`.
+
+```rust
+struct Empty;
+struct Color(i32, i32, i32);
+struct Length(i32);
+struct Status { Health: i32, Mana: i32, Attack: i32, Defense: i32 }
+struct HeightDatabase(Vec<i32>);
+```
+
+So you see that depending on the sub-datastructure, the `enum` variant, same as
+a struct, may or may not hold data. That is, in `Character`, `Digit` is a name
+tied to an `i32` where `Other` is just a name. However, the fact that they are
+distinct makes this very useful.
+
+As with structures, enums don't by default have access to operators such as
+compare ( `==` and `!=`), binary operations (`*` and `+`), and order
+(`<` and `>=`). As such, using the previous `Character` type, the
+following code is invalid:
+
+```{rust,ignore}
+// These assignments both succeed
+let ten  = Character::Digit(10);
+let four = Character::Digit(4);
+
+// Error: `*` is not implemented for type `Character`
+let forty = ten * four;
+
+// Error: `<=` is not implemented for type `Character`
+let four_is_smaller = four <= ten;
+
+// Error: `==` is not implemented for type `Character`
+let four_equals_ten = four == ten;
+```
+
+This may seem rather limiting, particularly equality being invalid; in
+many cases however, it's unnecessary. Rust provides the [`match`][match]
+keyword, which will be examined in more detail in the next section, which
+often allows better and easier branch control than a series of `if`/`else`
+statements would. However, for our [game][game] we need the comparisons
+to work so we will utilize the `Ordering` `enum` provided by the standard
+library which supports such comparisons. It has this form:
 
 ```{rust}
 enum Ordering {
@@ -211,14 +265,9 @@ enum Ordering {
 }
 ```
 
-An `Ordering` can only be _one_ of `Less`, `Equal`, or `Greater` at any given
-time.
-
-Because `Ordering` is provided by the standard library, we can use the `use`
-keyword to use it in our code. We'll learn more about `use` later, but it's
-used to bring names into scope.
-
-Here's an example of how to use `Ordering`:
+Because we did not define `Ordering`, we must import it (from the std
+library) with the `use` keyword. Here's an example of how `Ordering` is
+used:
 
 ```{rust}
 use std::cmp::Ordering;
@@ -245,11 +294,10 @@ fn main() {
 }
 ```
 
-There's a symbol here we haven't seen before: the double colon (`::`).
-This is used to indicate a namespace. In this case, `Ordering` lives in
-the `cmp` submodule of the `std` module. We'll talk more about modules
-later in the guide. For now, all you need to know is that you can `use`
-things from the standard library if you need them.
+The `::` symbol is used to indicate a namespace. In this case, `Ordering` lives
+in the `cmp` submodule of the `std` module. We'll talk more about modules later
+in the guide. For now, all you need to know is that you can `use` things from
+the standard library if you need them.
 
 Okay, let's talk about the actual code in the example. `cmp` is a function that
 compares two things, and returns an `Ordering`. We return either
@@ -259,95 +307,44 @@ the two values are less, greater, or equal. Note that each variant of the
 `Greater`.
 
 The `ordering` variable has the type `Ordering`, and so contains one of the
-three values. We can then do a bunch of `if`/`else` comparisons to check which
-one it is. However, repeated `if`/`else` comparisons get quite tedious. Rust
-has a feature that not only makes them nicer to read, but also makes sure that
-you never miss a case. Before we get to that, though, let's talk about another
-kind of enum: one with values.
+three values. We then do a bunch of `if`/`else` comparisons to check which
+one it is.
 
-This enum has two variants, one of which has a value:
+This `Ordering::Greater` notation is too long. Lets use `use` to import can
+the `enum` variants instead. This will avoid full scoping:
 
 ```{rust}
-enum OptionalInt {
-    Value(i32),
-    Missing,
-}
-```
-
-This enum represents an `i32` that we may or may not have. In the `Missing`
-case, we have no value, but in the `Value` case, we do. This enum is specific
-to `i32`s, though. We can make it usable by any type, but we haven't quite
-gotten there yet!
-
-You can also have any number of values in an enum:
+use std::cmp::Ordering::{self, Equal, Less, Greater};
 
-```{rust}
-enum OptionalColor {
-    Color(i32, i32, i32),
-    Missing,
-}
-```
-
-And you can also have something like this:
-
-```{rust}
-enum StringResult {
-    StringOK(String),
-    ErrorReason(String),
+fn cmp(a: i32, b: i32) -> Ordering {
+    if a < b { Less }
+    else if a > b { Greater }
+    else { Equal }
 }
-```
-Where a `StringResult` is either a `StringResult::StringOK`, with the result of
-a computation, or a `StringResult::ErrorReason` with a `String` explaining
-what caused the computation to fail. These kinds of `enum`s are actually very
-useful and are even part of the standard library.
 
-Here is an example of using our `StringResult`:
+fn main() {
+    let x = 5;
+    let y = 10;
 
-```rust
-enum StringResult {
-    StringOK(String),
-    ErrorReason(String),
-}
+    let ordering = cmp(x, y); // ordering: Ordering
 
-fn respond(greeting: &str) -> StringResult {
-    if greeting == "Hello" {
-        StringResult::StringOK("Good morning!".to_string())
-    } else {
-        StringResult::ErrorReason("I didn't understand you!".to_string())
-    }
+    if ordering == Less { println!("less"); }
+    else if ordering == Greater { println!("greater"); }
+    else if ordering == Equal { println!("equal"); }
 }
 ```
 
-That's a lot of typing! We can use the `use` keyword to make it shorter:
+Importing variants is convenient and compact, but can also cause name conflicts,
+so do this with caution. It's considered good style to rarely import variants
+for this reason.
 
-```rust
-use StringResult::StringOK;
-use StringResult::ErrorReason;
+As you can see, `enum`s are quite a powerful tool for data representation, and are
+even more useful when they're [generic][generics] across types. Before we
+get to generics, though, let's talk about how to use them with pattern matching, a
+tool that will let us deconstruct this sum type (the type theory term for enums)
+in a very elegant way and avoid all these messy `if`/`else`s.
 
-enum StringResult {
-    StringOK(String),
-    ErrorReason(String),
-}
-
-# fn main() {}
-
-fn respond(greeting: &str) -> StringResult {
-    if greeting == "Hello" {
-        StringOK("Good morning!".to_string())
-    } else {
-        ErrorReason("I didn't understand you!".to_string())
-    }
-}
-```
 
-`use` declarations must come before anything else, which looks a little strange in this example,
-since we `use` the variants before we define them. Anyway, in the body of `respond`, we can just
-say `StringOK` now, rather than the full `StringResult::StringOK`. Importing variants can be
-convenient, but can also cause name conflicts, so do this with caution. It's considered good style
-to rarely import variants for this reason.
-
-As you can see, `enum`s with values are quite a powerful tool for data representation,
-and can be even more useful when they're generic across types. Before we get to generics,
-though, let's talk about how to use them with pattern matching, a tool that will
-let us deconstruct this sum type (the type theory term for enums) in a very elegant
-way and avoid all these messy `if`/`else`s.
+[match]: ./match.html
+[game]: ./guessing-game.html#comparing-guesses
+[generics]: ./generics.html