Lisp/Variables
From Jonathan Gardner's Tech Wiki
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[edit] Variables
[edit] Lexical Variables
Two ways to bind variables: Function parameter definitions or LET.
(let ((x 1) (y 2)) body)
(LET is almost equivalent to a function declaration.)
LET* behaves like a nested let, one for each variable. This allows later variables to reference earlier variables.
(let* ((x 1) (y (*x 2))) body)
The equivalent code is:
(let ((x 1))
(let ((y (*x 2)))
body)
These are lexically scoped. That is, x is different inside of the LET than it is outside.
[edit] Closures
Variables referenced by functions created within a LET (or DEFUN) will, if returned, still have access to those variables even though the scope has been left.
[edit] Dynamic (aka Special, aka Global) Variables
Two ways: DEFVAR or DEFPARAMETER.
Convention dictates that globals should start and end with *, eg *global-variable*
[edit] DEFVAR
(defvar *var* value "documentation")
If the variable is already declared, does nothing. Otherwise, creates and initializes it. If you rebuild the code but want to keep the state, use a DEFVAR.
[edit] DEFPARAMETER
(defparameter *var* value "documentation")
Always overrides whatever is there, unlike DEFVAR. These are parameters that should be reset when the code is recompiled.
[edit] DEFCONSTANT
You can define constants with DEFCONSTANT. Convention dictates that you surround the name with '+', eg, "+constant-variable+".
Constants can, and will, be overridden. Use them only for things that are really constant like π.
[edit] Using Lexical to Override Global Temporarily
(let ((*standard-output* *some-other-stream*)) body)
Within body, *standard-output* is *some-other-stream*. Outside, it is what it would be otherwise.
[edit] Changing the Value
Use SETF to change the value.
(setf x 10) ; x = 10 (setf (aref a 0) 10) ; a[0] = 10 (setf (gethash 'key hash) 10) ; hash[key] = 10 (setf (field o) 10) ; o.field = 10
INCF and DECF are macros to make incrementing easier.
(incf x 10) ; x += 10 (decf x) ; x -= 1
ROTATEF swaps two values, and returns NIL.
(rotatef a b) ; a, b = b, a
SHIFTF slides the values to the left, and returns the left-most value.
(shiftf a b c 10) ; a, b, c = b, c, 10
