# Composing Subjects

So far, subjects have been composed by hand. To define their structure once, call the `NewSubjectScheme` function with a pattern, and use placeholders in braces for the variable parts:

```go
var bookSubject = architecturekit.NewSubjectScheme("/books/{book}")
```

The function returns a `*SubjectScheme`. To compose a subject, call the `Build` function with one value per placeholder, in the order in which they appear in the pattern. Use it in every command that acts on a book:

```go
func (c AcquireBook) Subject() string {
  return bookSubject.Build(c.BookID)
}

func (c BorrowBook) Subject() string {
  return bookSubject.Build(c.BookID)
}

func (c ReturnBook) Subject() string {
  return bookSubject.Build(c.BookID)
}
```

To take a subject apart, call the `Match` function. It returns the values by placeholder name, and `false` if the subject does not follow the pattern:

```go
values, ok := bookSubject.Match("/books/42")
// values["book"] == "42"
```

To get the pattern and the names of the placeholders, call the `Pattern` and the `Placeholders` function respectively.

*Note that a malformed pattern panics, as does calling `Build` with the wrong number of values, with an empty value, or with a value that contains a slash.*