LISP - Backquote and Comma

Modern Lisp: Backquote and Comma for Code Generation

Backquote (also known as quasiquote) is a fundamental tool in Lisp for constructing complex data structures, especially for generating code within macros. It acts like a ‘template engine’ for Lisp’s own data structures. Paired with comma and comma-at, it provides an elegant and readable way to build lists and code that would otherwise require convoluted calls to list, cons, and append.

The Core Trio: ``,,, and ,@`

Operator	Name	Purpose
(backquote)	Quasiquote	Creates a template. It quotes the expression that follows, but allows for specific parts to be ‘unquoted’.
, (comma)	Unquote	Escapes from the backquote. The expression following the comma is evaluated, and its result is inserted into the template.
,@ (comma-at)	Splicing Unquote	Evaluates the following expression (which must result in a list) and splices its elements into the surrounding list, rather than inserting the list itself.

Example: Basic Templating

Let’s see a simple example of building a list. We want to create the list (the value is 42) where 42 comes from a variable.

(let ((my-number 42))
  ;; Using backquote and comma is clean and readable
  (let ((templated-list `(the value is ,my-number)))
    (format t "Using backquote: ~s~%" templated-list))

  ;; For comparison, the equivalent using `list` is more verbose
  (let ((manual-list (list 'the 'value 'is my-number)))
    (format t "Using list:      ~s~%" manual-list)))

Output

Using backquote: (THE VALUE IS 42)
Using list:      (THE VALUE IS 42)

Explanation

• (the value is ,my-number): The backquote treats the, value, and is as literal symbols.
• ,my-number: The comma tells Lisp to stop quoting, evaluate the variable my-number (which yields 42), and insert the result into the list.

Example: Splicing with Comma-At (,@)

Now, let’s see how ,@ differs from ,. We want to insert the elements of one list into another.

(let ((inner-list '(b c d)))
  ;; Using comma-at splices the elements
  (format t "With ,@: ~s~%" `(a ,@inner-list e))

  ;; Using a regular comma inserts the list itself
  (format t "With , : ~s~%" `(a ,inner-list e)))

Output

With ,@: (A B C D E)
With , : (A (B C D) E)

The Primary Use Case: Writing Macros

The real power of backquote shines when writing macros. Macros are functions that transform code, and backquote is the standard tool for building the code to be generated. Let’s create a simple macro with-gensyms which is a common utility.

(defmacro with-gensyms ((&rest names) &body body)
  "A common macro that binds variables to fresh, uninterned symbols (gensyms).
   This helps avoid accidental variable capture in macros."
  `(let ,(mapcar (lambda (name) `(,name (gensym))) names)
     ,@body))

;; --- Macro Usage Example ---

;; We can now use our macro to create a `let` block with unique symbols.
;; Let's inspect the code our macro generates.
(let ((macro-expansion (macroexpand-1 '(with-gensyms (x y) (list x y)))))
  (format t "Macro Expansion:~%~s~%" macro-expansion))

;; When run, it will produce a list of two unique, temporary symbols.
(format t "~%Result of running the macro: ~s~%" (with-gensyms (x y) (list x y)))

Output

Macro Expansion:
(LET ((X (GENSYM)) (Y (GENSYM))) (LIST X Y))

Result of running the macro: (#:G1023 #:G1024)
;; Note: Your gensym names will be different.

Explanation of the Macro

• `(let ...)`: The outer backquote starts our code template. We are generating a let expression.
• ,(mapcar ...): The comma evaluates the mapcar expression. This expression builds the variable binding list for the let. For (x y), it generates ((X (GENSYM)) (Y (GENSYM))).
• ,@body: The comma-at splices the body of the macro call into the let block. If our macro was called with multiple forms, like (with-gensyms (x) (print x) (1+ x)), ,@ ensures they are all placed inside the let.