Lisp - Structure Types

In LISP, we can define basic structures using defstruct. In this chapter, we'll check features of structure types in details.

Basic Structures with defstruct

defstruct is a macro to create a simple structure with named slots. The new data type is created with automatically generated constructor and accessor functions.

Example - Create a Structure

main.lisp

; define a structure book
(defstruct book 
   title 
   author 
   subject 
   id 
)

; create book using make-book constructor and assign a structure to book1
( setq book1 (make-book :title "C Programming"
   :author "Nuha Ali" 
   :subject "C-Programming Tutorial"
   :id "478")
)

; get values from book1 using automatic accessors
(print(book-title book1))
(terpri)
(print(book-author book1))
(terpri)
(print(book-subject book1))
(terpri)
(print(book-id book1))

Output

When you execute the code, it returns the following result −

"C Programming" 
"Nuha Ali" 
"C-Programming Tutorial" 
"478" 

Key Features of LISP Structure

• Using defstruct, we can create a new data type.

• Accessor functions are generated automatically to access named slots.

• Constructor is generated automatically to create new instance.

• By this way, structure provides a basic data aggregation.

Limitations of LISP Structure

• A LISP Structure is simple as compared to full fledged CLOS (Common LISP Object System) object oriented data types.

• LISP Structure inheritance capability is limited. Only single inheritance is supported as compared to CLOS multiple inheritance. A LISP Structure inheritance is limited to inheriting the slots of included structure.

Conclusion

A LISP Structure is good for simple data grouping. We can use dynamic type checking of LISP to check types of structure making it suitable for custom data types.