Practical Common Lisp

132

CONS was originally short for the verb construct.

133

When the place given to SETF is a CAR or CDR, it expands into a call to the function RPLACA or RPLACD; some old- school Lispers—the same ones who still use SETQ—will still use RPLACA and RPLACD directly, but modern style is to use SETF of CAR or CDR.

134

Typically, simple objects such as numbers are drawn within the appropriate box, and more complex objects will be drawn outside the box with an arrow from the box indicating the reference. This actually corresponds well with how many Common Lisp implementations work—although all objects are conceptually stored by reference, certain simple immutable objects can be stored directly in a cons cell.

135

The phrase for-side-effect is used in the language standard, but recycling is my own invention; most Lisp literature simply uses the term destructive for both kinds of operations, leading to the confusion I'm trying to dispel.

136

The string functions NSTRING-CAPITALIZE, NSTRING- DOWNCASE, and NSTRING-UPCASE are similar—they return the same results as their N-less counterparts but are specified to modify their string argument in place.

137

For example, in an examination of all uses of recycling functions in the Common Lisp Open Code Collection (CLOCC), a diverse set of libraries written by various authors, instances of the PUSH/NREVERSE idiom accounted for nearly half of all uses of recycling functions.

138

There are, of course, other ways to do this same thing. The extended LOOP macro, for instance, makes it particularly easy and likely generates code that's even more efficient than the PUSH/ NREVERSE version.

139

This idiom accounts for 30 percent of uses of recycling in the CLOCC code base.

140

SORT and STABLE-SORT can be used as for-side-effect operations on vectors, but since they still return the sorted vector, you should ignore that fact and use them for return values for the sake of consistency.

141

NTH is roughly equivalent to the sequence function ELT but works only with lists. Also, confusingly, NTH takes the index as the first argument, the opposite of ELT. Another difference is that ELT will signal an error if you try to access an element at an index greater than or equal to the length of the list, but NTH will return NIL.

142

In particular, they used to be used to extract the various parts of expressions passed to macros before the invention of destructuring parameter lists. For example, you could take apart the following expression:

(when (> x 10) (print x))

Like this:

;; the condition
(cadr '(when (> x 10) (print x))) ==> (> X 10)
;; the body, as a list