# Defining State

The state holds what a command needs to decide on. Define it as a struct, call the `NewState` function with its initial value, and call the `Evolve` function for every event type that changes it:

```go
type Book struct {
  IsAcquired bool
  IsBorrowed bool
}

var bookState = architecturekit.NewState(Book{}).
  Evolve(func(book Book, event BookAcquired) Book {
    book.IsAcquired = true
    return book
  }).
  Evolve(func(book Book, event BookBorrowed) Book {
    book.IsBorrowed = true
    return book
  }).
  Evolve(func(book Book, event BookReturned) Book {
    book.IsBorrowed = false
    return book
  })
```

The event type is taken from the event's `EventType` function, so it does not have to be repeated.

*Note that calling `Evolve` twice for the same event type panics.*