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What would be an example of an anaphoric conditional in Lisp? Please explain the code as well.

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1  
I am only familiar with the term "anaphoric" as it pertains to natural languages -- for example, a pronoun can be considered an anaphoric reference to a previously mentioned noun. "John is a programmer. He works for Google." In this example, "he" is an anaphoric reference to "John". So I think you need to explain what an "anaphoric conditional" means in the context of Lisp programming -- it's not a commonly used term. –  Jim Lewis Oct 13 '10 at 1:42
 
@Jim_Lewis: Here's explanation what an "anaphoric conditional" means in the context of Lisp programming: weblogs.java.net/blog/2006/06/29/… –  blunders Oct 13 '10 at 1:52
2  
@Jim Lewis: Unsurprisingly, that's precisely what anaphoric means when referring to macros. See my answer for links and example. –  Greg Hewgill Oct 13 '10 at 1:55
 
Thanks for the links...I get it now! You learn something new every day around here... –  Jim Lewis Oct 13 '10 at 3:25
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2 Answers

up vote 3 down vote accepted

An example is the Common Lisp LOOP:

(loop for item in list
      when (general-predicate item)
      collect it)

The variable IT has the value of the test expression. This is a feature of the ANSI Common Lisp LOOP facility.

Example:

(loop for s in '("sin" "Sin" "SIN")
      when (find-symbol s)
      collect it)

returns

 (SIN)

because only "SIN" is a name for an existing symbol, here the symbol SIN. In Common Lisp symbol names have internally uppercase names by default.

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@Rainer_Joswig: Wow, thanks -- that was amazingly clear to me. Again, thank you!! –  blunders Oct 13 '10 at 12:05
 
@Rainer_Joswig: From your experience, what has been the best way to learn LISP for you? –  blunders Oct 13 '10 at 12:07
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up vote 8 down vote

Paul Graham's On Lisp has a chapter on Anaphoric Macros.

Essentially, it's a shorthand way of writing statements that avoids repeating code. For example, compare:

(let ((result (big-long-calculation)))
  (if result
      (foo result)))

and

(if (big-long-calculation)
    (foo it))

where it is a special name that refers to whatever was just calculated in (big-long-calculation).

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@Greg Hewgill: Thanks, still sort of lost in Paul Graham's code, but it's fun to process. –  blunders Oct 13 '10 at 2:11
2  
@blunders: Indeed. Anaphoric macros probably isn't the best place to start, unless you're already very familiar with the different styles of macros in Lisp. On Lisp is a great read. –  Greg Hewgill Oct 13 '10 at 2:19
 
@Greg Hewgill: Agree. I've scanned a few LISP books, mainly The Little Schemer, and the like. "Anaphoric conditional" randomly came up in doing research for this question: stackoverflow.com/questions/3920046/… –  blunders Oct 13 '10 at 2:26
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