RustBrock/Error Handling.md

# Error Handing

This is a factor of life in software,
Rust has a number of features for handling errors. One feature is that Rust requires you to acknowledge the possibility of an error and take some action beofre our code will compile.

This requirement ensures that errors are handled before the possiblity could arise

This can be split into two major categories
 - [*Recoverable*](#recoverable-errors) - File a file not found, just need to report the problem to the user and retry the operation
 - [*Unrecoverable*](#unrecoverable-errors) - A symptom of bugs, like trying to access a location beyond the end of an array. Need to immediately stop the program

Many languages dont distinguish between the two kinds of errors and handle them the same way using mechanisms such as exceptions

Rust does not have exceptions

Instead it has the type `Result< T, E>` for recoverable errors

It has the `panc!` macro to stop eecution when an unrecoverable error occurs

## Unrecoverable Errors
Whne bad things happen in your code and nothing you can do nothing about it then Rust has the `panc!` macro

There are two ways to cause a panic:
- by taking an action that causes the code to paic (like accessing an array past the end)
- explicity calling `panic!` macro

By default these print a failure message, unwind, clean up the stack and then quit.

Using an environment variable you can also have Rust display the call stack when a panic occurs. This can make it easier to track down the source of the panic

When a call to `panic!` occurs the error message will be contained in the last two lines. The first line will contain our message and the second is when te source of this panic occured

example
```rust
fn main() {
    panic!("crash and burn");
}
```

This will output
```
thread 'main' panicked at src/main.rs:2:5:
crash and burn
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

This indicates that the panic occured in the file main.rs at the 2nd line on the 5th character

In this example it indicates that it is part of our source cdoe, looking there will show the `panic!` macro

In other cases the `panic!` call might be reported as someone else code where the `panic!` macro was called

You can also use the traceback functions of the `panic` call to figure ot the part of our code that caused the problem

To understand this an example will be used

```rust
fn main() {
    let v = vec![1, 2, 3];

    v[99];
}
```

Here we are tryin to access the 100th element, this is out of range and therefore Rust will initiate a error

In C, attempting to read beyond hte end of a data structure is undefined behavior, and you might get whatever is at the memory location, this would be something "random"

This is considered a *buffer overread* and can lead to security vulnerabilities, this would allow an attacker to be able to manipulate the index in such a way that they shouldnt be allowed to sore in that data structure.

Rust protects yo from this kind of vulnerability by casuing a panic if you try to read something out of range.

The `note:` line tells us that we can set the `RUST_BACKTRACE` environment variable to get a backtrace to show exactly what happened to casue the error. 

The key to reading a backtrace is to start at the top and read until you see the files you wrote, that is where the problem originates.

The lines above that spot are code that our code has called, and the lines below are the code that called your code.
These before-and-after lines might include core Rust code, std lib code or crates that you are using

You can set the backtrace by setting the `RUST_BACKTRACE` environment variable to any value except 0

Example
```
RUST_BACKTRACE=1 cargo run
thread 'main' panicked at src/main.rs:4:6:
index out of bounds: the len is 3 but the index is 99
stack backtrace:
   0: rust_begin_unwind
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:645:5
   1: core::panicking::panic_fmt
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/panicking.rs:72:14
   2: core::panicking::panic_bounds_check
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/panicking.rs:208:5
   3: <usize as core::slice::index::SliceIndex<[T]>>::index
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/slice/index.rs:255:10
   4: core::slice::index::<impl core::ops::index::Index<I> for [T]>::index
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/slice/index.rs:18:9
   5: <alloc::vec::Vec<T,A> as core::ops::index::Index<I>>::index
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/alloc/src/vec/mod.rs:2770:9
   6: panic::main
             at ./src/main.rs:4:6
   7: core::ops::function::FnOnce::call_once
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
```

In order to a backtarace with this info, debug symbols must be enabled

Debug symbols are enabled by defualt when using `cargo build` or `cargo run` without the `--release` flag

In line 6 of the backtrace points to the line in our project that causes the problem, that would be line 4 of *src/main.rs* 

If we dont want our program to panc thne we sould start our investigation at the inidcated line we wrote

### Unwinding the Stack or Aborting in Response to a Panic
*unwinding* in rust means that it walks back up the stack and cleans up the data form each function it encounters.

However walking back and cleaning up is a lot of work

Rust also allows yo to choose the alternative of immediately *aborting*, which means ends the program without cleaning up

Memory that the program was using will thne be clean up by the OS

If yo need your project's resultant binary as small as possible you can switch from unwinfing to aborting upon a panic

This can be done by adding `panic = 'abort'` to the appropriate `[profile]` section in your Cargo.toml

example of this
```toml
[profile.release]
panic = 'abort'
```

## Recoverable Errors

You can use the enum `Result` to handle most errors becasue they are not serious enough to warrant a panic

One example of this of a non serious error is opening a file and that operation fails becasue that file doesnt exist, you may want to create the file instead of terminating the process

the enum `Result` is defined as
Which has two variants `Ok` and `Err`

```rust
enum Result<T, E> {
    Ok(T),
    Err(E),
}
```

`T` and `E` are generic type parameters

`T` reperesents the tpye of value that will be returned in a success case within th `Ok` variant
`E` reperesents the tpye of the error that will be returned in a failure case within the `Err` variant

Because `Result` has these generic type parameters we can use the `Result` type and the functions defined on it in many different situations where the success value and error value we want to reutrn may differ

Lets use a function that retunrs a `Result` value because the function could fail

```rust
use std::fs::File;

fn main () {
    let greeting_file_result = File::open("hello.txt");
}
```

The return type of `File::open` is a `Result<T, E>`
The generic parameter `T` has been filled in by the implementation of `File::open` with the type of the success value is a file handle (`std::fs::File`)
The `E` parameter is used in the error value which is `std::io::Error`

This return type indicates that call may succeed and reutnr a file handle that we can read and write to or it may fail if it doesnt exist, or not having the correct permissions

`File::open` function needs a way to tell us whether it succeeded or failed hence the use of `Result` enum which conveys this message of failure or success

When `File::open` succeeds, the value in `greeting_file_result` will be an instance of `Ok` that contains a file handle
When it fails the value in `greeting_file_result` will be an intance of `Er` that contains more info about the kind of error that occurred

We need to add a match in order to use `Result` here is one option

```rust
use std::fs::File;

fn main() {
    let greeting_file_result = File::open("hello.txt");

    let greeting_file = match greeting_file_result {
        Ok(file) => file,
        Err(error) => panic!("Problem opening the file: {error:?}"),
    };
}
```

This is like the `Option` enum
`Result` enum and its variants have been brought into scope by the prelude, so you dont need to specifiy `Result::` before the `Ok` and `Err` variants in the match arm

When the result is `Ok` the code will reutrn the `file` value out itself and then we can assign that file handle to the `greeting_file`. After machh we can use the file for reading or writing.

The other arm in `match` handles the `Err` value we get from `File::open`, in this case we choose to call the `panic!` macro with more details about the error

### Matching on Different Errors

You can take different actions depending on the type of failure

For example if a file doesnt open because it doesnt exist then maybe you want you want to create the file first instead of panicing and exiting the program

One way we can do this is by using a `match` for the returned value `Result<T, E>` 

Example
```rust
use std::fs::File;
use std::io::ErrorKind;

fn main() {
    let greeting_file_result = File::open("hello.txt");

    let greeting_file = match greeting_file_result {
        Ok(file) => file,
        Err(error) => match error.kind() {
            ErrorKind::NotFound => match File::create("hello.txt") {
                Ok(fc) => fc,
                Err(e) => panic!("Problem creating the file: {e:?}"),
            },
            other_error => {
                panic!("Problem opening the file: {other_error:?}");
            }
        },
    };
}
```

The tpye that ``File::open` reutrn inside thr `Err` varian is `io:Error`, this is a struct provided by the std library

This struct has a method `kind` which can be called to get an `io::ErrorKind` value

The enum `io::ErrorKind` is also provided by a std library which has vairants representing the different kinds of errors that may result from an `io` operation

The error we care about is `Error::NotFound`, this indicates that the file that we try to open doesnt exist.

In this case we need to have a match on the outher thne use that on an inner match on `error.kind()`

In this inner match we create a file which can also fail hence in 2nd inner match to each return a file handle or panic out of the program

### Alternatives to Using match with `Result<T, E>`

this match expression is very useful but also primitive and boilerplate

one example to condese this code is

```rust
use std::fs::File;
use std::io::ErrorKind;

fn main() {
    let greeting_file = File::open("hello.txt").unwrap_or_else(|error| {
        if error.kind() == ErrorKind::NotFound {
            File::create("hello.txt").unwrap_or_else(|error| {
                panic!("Problem creating the file: {error:?}");
            })
        } else {
            panic!("Problem opening the file: {error:?}");
        }
    });
}
```

This accomplishes the same thing as the the example above but it uses if and else and the `unwrap_or_else`

this is much cleaner to read even though it odesnt use and match statements

look at the exact definition `unwrap_or_else` in the std library document

### Shortcuts for Panic on Error: unwrap and expect

`match` works well, but it can be a bit verbose and doesnt communicate clearly what the intent is

`Result<T, E>` has many helper methods to do specific tasks

The `unwrap` method that is a shortcut for the `match` expression that was used before

If the `Result` value is a `Ok` variant, `unwrap` will return the value inside the `Ok`

If the `Result` is the `Err` variant, `unwrap` will call the `panc!` macro for us

Here is an example of its use
```rust
use std::fs::File;

fn main() {
    let greeting_file = File::open("hello.txt").unwrap();
}
```

If this code without a *hello.txt* it will panic with an error like this
```
thread 'main' panicked at src/main.rs:4:49:
called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }
```

You can also use the `expect` method lets us also choose the `panic!` error message

This is good for providing clearer error mesages and make tracking down the source of the panic easier

Here is an example of an `expect` in use
```rust
use std::fs::File;

fn main() {
    let greeting_file = File::open("hello.txt")
        .expect("hello.txt should be included in this project");
}
```

`expect` is used in the same way as `unwrap` to retunr the file handle or call the `panic!` macro

The message is used by `expect` in its call to `panic!` as the paramter that is passed to `expect` rather than a defualt message

`expect` is maninly used in production-quality code over `unwrap` becasue it gives more context about why the opertaion is always expected to succeed

### Propagating Errors
When a operation fails you may want to pass back the error instead of handling the error within the function itself