Common LISP Hints: Structures

## 15. Structures

LISP structures are analogous to C structs or Pascal records. Here is an example:

> `

>     > (defstruct foo
>         bar
>         baaz
>         quux)
>     FOO

> `

This example defines a data type called `foo` which is a structure containing 3 fields. It also defines 4 functions which operate on this data type: `make-foo`, `foo-bar`, `foo-baaz`, and `foo-quux`. The first one makes a new object of type `foo`; the others access the fields of an object of type `foo`. Here is how to use these functions:

> `

>     > (make-foo)
>     #s(FOO :BAR NIL :BAAZ NIL :QUUX NIL)
>     > (make-foo :baaz 3)
>     #s(FOO :BAR NIL :BAAZ 3 :QUUX NIL)
>     > (foo-bar *)
>     NIL
>     > (foo-baaz **)
>     3

> `

The `make-foo` function can take a keyword argument for each of the fields a structure of type `foo` can have. The field access functions each take one argument, a structure of type `foo`, and return the appropriate field.

See below for how to set the fields of a structure.