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Forward back propagate changes to the nostarch md

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Carol (Nichols || Goulding) 2017-01-27 17:28:53 -05:00
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nostarch/chapter06.md
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@ -59,7 +59,7 @@ Note that the variants of the enum are namespaced under its identifier, and we
use a double colon to separate the two. The reason this is useful is that now use a double colon to separate the two. The reason this is useful is that now
both values `IpAddrKind::V4` and `IpAddrKind::V6` are of the same type: both values `IpAddrKind::V4` and `IpAddrKind::V6` are of the same type:
`IpAddrKind`. We can then, for instance, define a function that takes any `IpAddrKind`. We can then, for instance, define a function that takes any
`IpAddrKind` as an argument: `IpAddrKind`:
```rust ```rust
fn route(ip_type: IpAddrKind) { } fn route(ip_type: IpAddrKind) { }
@ -104,7 +104,7 @@ Listing 6-1: Storing the data and `IpAddrKind` variant of an IP address using a
`struct` `struct`
</caption> </caption>
Here, we’ve defined a struct `IPAddr` that has two fields: a `kind` field that Here, we’ve defined a struct `IpAddr` that has two fields: a `kind` field that
is of type `IpAddrKind` (the enum we defined previously) and an `address` field is of type `IpAddrKind` (the enum we defined previously) and an `address` field
of type `String`. We have two instances of this struct. The first, `home`, has of type `String`. We have two instances of this struct. The first, `home`, has
the value `IpAddrKind::V4` as its `kind` with associated address data of the value `IpAddrKind::V4` as its `kind` with associated address data of
@ -224,9 +224,10 @@ struct WriteMessage(String); // tuple struct
struct ChangeColorMessage(i32, i32, i32); // tuple struct struct ChangeColorMessage(i32, i32, i32); // tuple struct
``` ```
But if we used the different structs, we wouldn’t be able to as easily define a But if we used the different structs, which each have their own type, we
function that could take any of these kinds of messages as we could with the wouldn’t be able to as easily define a function that could take any of these
`Message` enum, defined in Listing 6-2. kinds of messages as we could with the `Message` enum defined in Listing 6-2,
which is a single type.
There is one more similarity between enums and structs: just as we’re able to There is one more similarity between enums and structs: just as we’re able to
define methods on structs using `impl`, we’re also able to define methods on define methods on structs using `impl`, we’re also able to define methods on
@ -320,7 +321,8 @@ let absent_number: Option<i32> = None;
``` ```
If we use `None` rather than `Some`, we need to tell Rust what type of If we use `None` rather than `Some`, we need to tell Rust what type of
`Option<T>` we have. `Option<T>` we have, because the compiler can't infer the type that the `Some`
variant will hold by looking only at a `None` value.
When we have a `Some` value, we know that a value is present, and the value is When we have a `Some` value, we know that a value is present, and the value is
held within the `Some`. When we have a `None` value, in some sense, it means held within the `Some`. When we have a `None` value, in some sense, it means
@ -604,7 +606,7 @@ Now let’s consider the second call of `plus_one` in Listing 6-5 where `x` is
None => None, None => None,
``` ```
It matches! There’s no value to add to, so the program stops and returns the It matches! There’s no value to add to, so the program stops and returns the
`None` value on the right side of `=>`. Because the first arm matched, no other `None` value on the right side of `=>`. Because the first arm matched, no other
arms are compared. arms are compared.