floor, truncate, and related functions can take an optional second argument to use as the divisor. The default value is the integer 1. Instead of (floor (/ x 42)) you can use (floor x 42).
There are two means of automatic initialization of CLOS slot values, :initform and :default-initargs. Chris Riesbeck makes a strong case for preferring :default-initargs.
Here’s a simplified example of the syntactic difference, taken from some Quicklisp code. Consider modeling version control commands with CLOS. Here’s a possible design:
(defclass vc-command () ((command :initarg :command :accessor command) (checkout-subcommand :initarg :checkout-subcommand :accessor checkout-subcommand)))
To make subclasses for CVS and git, one option is to use the initform slot option:
(defclass vc-git (vc-command) ((command :initform "git") (checkout-subcommand :initform "clone"))) (defclass vc-cvs (vc-command) ((command :initform "cvs") (checkout-subcommand :initform "co")))
I prefer to use :default-initargs to avoid recapping the slots:
(defclass vc-git (vc-command) () (:default-initargs :command "git" :checkout-subcommand "clone")) (defclass vc-cvs (vc-command) () (:default-initargs :command "cvs" :checkout-subcommand "co"))
:default-initargs also need not correspond to slots. They can be used to pass extra arguments to initialization functions like initialize-instance:
(defclass fribble (dibble) () (:default-initargs :persist nil)) (defmethod initialize-instance :after ((instance dibble) &key persist) (when persist (persist-object instance)))
floor, truncate, and related functions return multiple values for division-related operations: the quotient, and the remainder. There are several situations where both values are useful.
For example, if you have a vector of octet values and you want to find the value of a particular bit, you can use something like this:
(defun bit-ref (octet-vector index) (multiple-value-bind (octet-index bit-index) (truncate index 8) (ldb (byte 1 bit-index) (aref octet-vector octet-index))))
Another use is copying a known amount of data in fixed-size chunks, e.g. when copying a file somewhere. The quotient represents the number of full chunks occupied by the data, and the remainder is the count of elements in the final partial chunk.
The Unix epoch begins at 00:00 on January 1, 1970, GMT. time() returns the number of seconds elapsed since then.
It’s easy to get the difference between a Common Lisp universal time and a Unix epoch timestamp:
* (encode-universal-time 0 0 0 1 1 1970 0) 2208988800
When working with Unix epoch timestamp data, it’s easy to write conversion functions:
(defvar *unix-epoch-difference* (encode-universal-time 0 0 0 1 1 1970 0)) (defun universal-to-unix-time (universal-time) (- universal-time *unix-epoch-difference*)) (defun unix-to-universal-time (unix-time) (+ unix-time *unix-epoch-difference*)) (defun get-unix-time () (universal-to-unix-time (get-universal-time)))
Comparing CL output to the shell:
* (get-unix-time) 1318685294
$ date +%s 1318685294
(My CL implementation doesn’t constant-fold the call to encode-universal-time. Bummer.)
I sometimes see a question like this: “I have a list of things and I want to see if they’re all true, so how can I apply ‘and’ to it?” Since and is a macro, it can’t be applied, but there are standard functions that can answer the question instead.
Sometimes you know in advance how you want to change control flow in a loop. Other times you might want to defer that decision by offering a restart. For example, here’s a loop with a pair of with-simple-restarts:
(with-simple-restart (stop-processing "Stop processing users") (dolist (user (pending-user-list)) (with-simple-restart (skip "Skip user ~A" user) (process-user user))))
If there is a problem processing users, you might get something in the debugger that looks like this:
Could not find home directory for "nancy" [Condition of type SIMPLE-ERROR] Restarts: 0: [SKIP] Skip user nancy 1: [STOP-PROCESSING] Stop processing users
You can then interactively choose which action to take, depending on what’s most appropriate for the condition at hand.
(You can also choose restarts non-interactively. That’s a tip for another day.)
Producing “pretty” output can be time consuming. Some implementations do a lot of work to determine whether to e.g. break lines when printing with functions like print and format.
If printing is a bottleneck and aesthetic output isn’t required, binding or setting *print-pretty* to NIL can significantly improve output speed for some implementations.
To assign the multiple return values of a function to multiple variables, you could use this:
(multiple-value-setq (whole partial) (truncate x 1024))
(setf values) is more general, and works on places:
(setf (values whole partial) (truncate x 1024))
(setf (values (aref v 0) (aref v 1)) (truncate x 1024))
The “do” macros (do, do*, dolist, and dotimes) have bodies that act like tagbody. You can put go tags anywhere in the body and use go to jump to them from arbitrary places. This can be useful for skipping, retrying, or otherwise changing the flow of iteration.
(dolist (users (get-user-list)) :retry ... (when some-condition (go :retry) ...)
The numeric comparison functions =, /=, <, <=, >, and >= can take more than two arguments. This is handy to e.g. check if several variables have monotonically increasing numeric value:
(< a b c d)
For numbers in a sufficiently short list, apply does the trick:
(apply #'< list)
Non-numeric comparison functions generally take exactly two objects to compare, so e.g. (string= x y z) is invalid, and for a list of three or more objects, you can’t just apply the function to the list.
(every #'string= list (rest list))
Note that /= is special; the following calls are not equivalent:
* (apply #'/= list) T
* (every #'/= list (rest list)) T