Anaphoric macro

Ananaphoric macrois a type ofprogramming macrothat deliberately captures some form supplied to the macro which may be referred to by ananaphor(an expression referring to another). Anaphoric macros first appeared inPaul Graham'sOn Lispand their name is a reference tolinguistic anaphora—the use of words as a substitute for preceding words.

Examples

Theloopmacro inANSI Common Lispis anaphoric in binding, where theitexpression refers to the result of the test expression in a clause.

Here is an example that sums the value of non-nilelements, whereitrefers to the values of elements that do not equalnil:

(loopforelementin'(nil1nil2nilnil346)
whenelementsumit)
;; ⇒ 16

Hereitis bound to the output of(and (> number 3) number)when true, collecting numbers larger than 3:

(loopfornumberfrom1to6
when(and(>number3)number)
collectit);IT refers to (and (> number 3) number).
;; ⇒ (4 5 6)

Defining anaphoric macros

One example is an anaphoric version of theif-then-else construct,which introduces ananaphorit,bound to the result of the test clause:

(defmacroaif(test-formthen-form&optionalelse-form)
`(let((it,test-form))
(ifit,then-form,else-form)))

(aif(+27)
(formatnil"~A does not equal NIL."it)
(formatnil"~A does equal NIL."it))
;; ⇒ "9 does not equal NIL."

Another example is an anaphoric version of theλ-function,which binds the function itself to theanaphorself,allowing it torecur:

(defmacroalambda(parms&bodybody)
`(labels((self,parms,@body))
#'self))

;; Factorial function defined recursively where `self' refers to the alambda function
(alambda(n)
(if(=n0)
1
(*n(self(1-n)))))

See also

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

External links

• Chapter 14. Anaphoric MacrosfromOn LispbyPaul Graham
• Anaphora— an anaphoric macro collection