Making Decisions

var acquireBook = architecturekit.Decider[AcquireBook, Book]{
  State: bookState,
  Decide: func(ctx context.Context, cmd AcquireBook, book Book) ([]architecturekit.Event, error) {
    if book.IsAcquired {
      return nil, architecturekit.NewDomainError("book %s has already been acquired", cmd.BookID)
    }

    return []architecturekit.Event{
      BookAcquired{
        Title:  cmd.Title,
        Author: cmd.Author,
        ISBN:   cmd.ISBN,
      },
    }, nil
  },
}

To reject a command, return an error created with the NewDomainError function. It takes a format string and arguments, like fmt.Errorf, and returns a *DomainError, whose message is exactly the formatted text, and which belongs to the category ErrDomain (see Handling Errors).

A decider may check several rules:

var borrowBook = architecturekit.Decider[BorrowBook, Book]{
  State: bookState,
  Decide: func(ctx context.Context, cmd BorrowBook, book Book) ([]architecturekit.Event, error) {
    if !book.IsAcquired {
      return nil, architecturekit.NewDomainError("book %s does not exist", cmd.BookID)
    }
    if book.IsBorrowed {
      return nil, architecturekit.NewDomainError("book %s is already borrowed", cmd.BookID)
    }

    return []architecturekit.Event{
      BookBorrowed{
        BorrowedBy:    cmd.ReaderID,
        BorrowedUntil: cmd.BorrowedUntil,
      },
    }, nil
  },
}

If there is nothing to do, return neither events nor an error:

var returnBook = architecturekit.Decider[ReturnBook, Book]{
  State: bookState,
  Decide: func(ctx context.Context, cmd ReturnBook, book Book) ([]architecturekit.Event, error) {
    if !book.IsBorrowed {
      return nil, nil
    }

    return []architecturekit.Event{
      BookReturned{},
    }, nil
  },
}