src/ch06-03-if-let.md

## Concise Control Flow with `if let` and `let else`

The `if let` syntax lets you combine `if` and `let` into a less verbose way to
handle values that match one pattern while ignoring the rest. Consider the
program in Listing 6-6 that matches on an `Option<u8>` value in the
`config_max` variable but only wants to execute code if the value is the `Some`
variant.

<Listing number="6-6" caption="A `match` that only cares about executing code when the value is `Some`">

```rust
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-06/src/main.rs:here}}
```

</Listing>

If the value is `Some`, we print out the value in the `Some` variant by binding
the value to the variable `max` in the pattern. We don’t want to do anything
with the `None` value. To satisfy the `match` expression, we have to add `_ =>
()` after processing just one variant, which is annoying boilerplate code to
add.

Instead, we could write this in a shorter way using `if let`. The following
code behaves the same as the `match` in Listing 6-6:

```rust
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-12-if-let/src/main.rs:here}}
```

The syntax `if let` takes a pattern and an expression separated by an equal
sign. It works the same way as a `match`, where the expression is given to the
`match` and the pattern is its first arm. In this case, the pattern is
`Some(max)`, and the `max` binds to the value inside the `Some`. We can then
use `max` in the body of the `if let` block in the same way we used `max` in
the corresponding `match` arm. The code in the `if let` block only runs if the
value matches the pattern.

Using `if let` means less typing, less indentation, and less boilerplate code.
However, you lose the exhaustive checking that `match` enforces. Choosing
between `match` and `if let` depends on what you’re doing in your particular
situation and whether gaining conciseness is an appropriate trade-off for
losing exhaustive checking.

In other words, you can think of `if let` as syntax sugar for a `match` that
runs code when the value matches one pattern and then ignores all other values.

We can include an `else` with an `if let`. The block of code that goes with the
`else` is the same as the block of code that would go with the `_` case in the
`match` expression that is equivalent to the `if let` and `else`. Recall the
`Coin` enum definition in Listing 6-4, where the `Quarter` variant also held a
`UsState` value. If we wanted to count all non-quarter coins we see while also
announcing the state of the quarters, we could do that with a `match`
expression, like this:

```rust
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-13-count-and-announce-match/src/main.rs:here}}
```

Or we could use an `if let` and `else` expression, like this:

```rust
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-14-count-and-announce-if-let-else/src/main.rs:here}}
```

## Staying on the “Happy Path” with `let...else`

One common pattern is to perform some computation when a value is present and
return a default value otherwise. Continuing on with our example of coins with a
`UsState` value, if we wanted to say something funny depending on how old the
state on the quarter was, we might introduce a method on `UsState` to check the
age of a state, like so:

```rust
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-07/src/main.rs:state}}
```

Then we might use `if let` to match on the type of coin, introducing a `state`
variable within the body of the condition, as in Listing 6-7.

<Listing number="6-7" caption="Checking whether a state existing in 1900 by using conditionals nested inside an `if let`." file-name="src/main.rs">

```rust
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-07/src/main.rs:describe}}
```

</Listing>

That gets the job done, but it has pushed the work into the body of the `if let`
statement, and if the work to be done is more complicated, it might be hard to
follow exactly how the top-level branches relate. We could also take advantage
of the fact that expressions produce a value either to produce the `state` from
the `if let` or to return early, as in Listing 6-8. (You could do similar with a
`match`, too.)

<Listing number="6-8" caption="Using `if let` to produce a value or return early." file-name="src/main.rs">

```rust
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-08/src/main.rs:describe}}
```

</Listing>

This is a bit annoying to follow in its own way, though! One branch of the `if
let` produces a value, and the other one returns from the function entirely.

To make this common pattern nicer to express, Rust has `let...else`. The
`let...else` syntax takes a pattern on the left side and an expression on the
right, very similar to `if let`, but it does not have an `if` branch, only an
`else` branch. If the pattern matches, it will bind the value from the pattern
in the outer scope. If the pattern does _not_ match, the program will flow into
the `else` arm, which must return from the function.

In Listing 6-9, you can see how Listing 6-8 looks when using `let...else` in
place of `if let`. Notice that it stays “on the happy path” in the main body of
the function this way, without having significantly different control flow for
two branches the way the `if let` did.

<Listing number="6-9" caption="Using `let...else` to clarify the flow through the function." file-name="src/main.rs">

```rust
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-09/src/main.rs:describe}}
```

</Listing>

If you have a situation in which your program has logic that is too verbose to
express using a `match`, remember that `if let` and `let...else` are in your
Rust toolbox as well.

## Summary

We’ve now covered how to use enums to create custom types that can be one of a
set of enumerated values. We’ve shown how the standard library’s `Option<T>`
type helps you use the type system to prevent errors. When enum values have
data inside them, you can use `match` or `if let` to extract and use those
values, depending on how many cases you need to handle.

Your Rust programs can now express concepts in your domain using structs and
enums. Creating custom types to use in your API ensures type safety: the
compiler will make certain your functions only get values of the type each
function expects.

In order to provide a well-organized API to your users that is straightforward
to use and only exposes exactly what your users will need, let’s now turn to
Rust’s modules.