> For the complete documentation index, see [llms.txt](https://openai.gitbook.io/code-cheatsheets/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://openai.gitbook.io/code-cheatsheets/all/elixir/errors.md).

# Errors

Elixir has three error mechanisms: errors, throws, and exits.

Instead of all these constructs, Elixir's supervisor system is better to just fail fast and restart the app in a known state

```elixir
foo + 1 # (ArithmeticError) bad argument in arithmetic expression :erlang.+(:foo, 1)

raise "oops" # (RuntimeError) oops

raise ArgumentError, message: "invalid argument foo" # (ArgumentError) invalid argument foo
```

## Errors

Errors (or *exceptions*) are used when exceptional things happen in the code.

In Elixir, we avoid using `try/rescue` because **we don’t use errors for control flow**. Try/catch => try/rescue, in practice rarely used b/c tuple with err often used instead

```elixir
err = try do
    5/0
rescue
    ArithmeticError -> "Can't Divide by Zero"
end
IO.puts err
```

### Custom Errors

```elixir
defmodule MyError do
    defexception message: "default message"
end

iex> raise MyError
** (MyError) default message
iex> raise MyError, message: "custom message"
** (MyError) custom message
```

## Throws

`throw` and `catch` are reserved for situations where it is not possible to retrieve a value unless by using `throw` and `catch`.

Also uncommon

## Exits

When a process dies of "natural causes" (unhandled exceptions), it sends an exit signal