# Saying what can fail: `fails`

A function or action lists the failures it can end with after `fails`, in
its signature:

```jadpo
function check_title(title: TodoTitle) fails TitleBlank -> TodoTitle {
    if (title == "") {
        reject TitleBlank
    }
    return title
}

action rename(todo_id: Todo.id, title: TodoTitle) fails TodoNotFound, TitleBlank -> TodoView {
    var checked: TodoTitle = attempt check_title(title)
    var todo: Todo = attempt Todo.get(todo_id)
    ...
}
```

`fails` comes before the result type's `->`
([Signatures and settings](functions-and-actions.md#signatures-and-settings)). It
lists exactly the failures that can leave the callable: everything a
`reject` or an `attempt` in its body can produce, less what a `match`
handles. Functions and actions follow the same rule.

- A failure that can leave but isn't listed is
  `failure.undeclared_propagation`.
- One that is listed but can never happen is `failure.stale_declaration`.
- One listed twice is `failure.duplicate_declaration`.

[Faults](faults.md) are never listed.