Anaphoric Macro

An anaphoric macro is a type of programming macro that deliberately captures some form supplied to the macro which may be referred to by an ''anaphor'' (an expression referring to another). Anaphoric macros first appeared in Paul Graham's '' On Lisp''Chapter 6
of Let over Lambda and their name is a reference to linguistic anaphora—the use of words as a substitute for preceding words.

Examples

The loop macro in ANSI Common Lisp is anaphoric in binding, where the it expression refers to the result of the test expression in a clause.

Here is an example that sums the value of non-nil elements, where it refers to the values of elements that do not equal nil:

(loop for element in '(nil 1 nil 2 nil nil 3 4 6)
when element sum it)
;; ⇒ 16

Here it is bound to the output of (and (> number 3) number) when true, collecting numbers larger than 3:6.1.8.1 Examples of clause grouping
from the Common Lisp HyperSpec

(loop for number from 1 to 6
when (and (> number 3) number)
collect it) ; IT refers to (and (> number 3) number).
;; ⇒ (4 5 6)

Defining anaphoric macros

One example is an anaphoric version of the if-then-else construct, which introduces an ''anaphor'' it, bound to the result of the test clause:Chapter 14. Anaphoric Macros
of On Lisp by Paul Graham

(defmacro aif (test-form then-form &optional else-form)
`(let ((it ,test-form))
(if it ,then-form ,else-form)))

(aif (+ 2 7)
(format nil "~A does not equal NIL." it)
(format nil "~A does equal NIL." it))
;; ⇒ "9 does not equal NIL."

Another example is an anaphoric version of the λ-function, which binds the function itself to the ''anaphor'' self, allowing it to recur:

(defmacro alambda (parms &body body)
`(labels ((self ,parms ,@body))
#'self))

;; Factorial function defined recursively where `self' refers to the alambda function
(alambda (n)
(if (= n 0)
1
(* n (self (1- n)))))

See also

* Anonymous recursion
* Hygienic macros
* Macro (computer science)
* Method chaining
* this (computer programming)

References

External links

{{wikibooks, Common Lisp, External libraries/Extended Binding#Anaphoric macros with Anaphora, Anaphoric macros with Anaphora

Chapter 14. Anaphoric Macros
from On Lisp by Paul Graham

Anaphora
— an anaphoric macro collection

Lisp (programming language)
Programming constructs
Source code
Articles with example Lisp (programming language) code