What would be an example of an anaphoric conditional in Lisp?

问题

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

回答1:

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.

回答2:

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).

来源：https://stackoverflow.com/questions/3920193/what-would-be-an-example-of-an-anaphoric-conditional-in-lisp