Basic definitions: lists, functions, and evaluation.
The Lisp forms tree
/ | \
___Rational___ Float Complex
- A Lisp expression may be an atom (e.g. numeric constant, symbol, etc.) or a list (a collection of atoms and lists bracketed by parenthesis). Anything that is not a list is an atom.
Symbols and Special Forms
- Lisp symbols allows us to label data with names. You can use almost all the characters of the keyboard to create a symbol.
- Since most Lisp interpreters are case insensitive, Lisp programmers use the - character to separate words in symbols:
- Symbols are initially unbound (have no value), except for special pre-defined symbols. Try to evaluate this unbound symbol:
- Things goes different if you try to evaluate this pre-bound symbol:
- The defvar macro can be used to define a global variable and bind it to a value:
(defvar A-4 440)
Note that defvar evaluates to its first argument. Now, if we evaluate our new variable:
- Defvar is a special form it does not follow the usual rules of evaluation. Specifically, defvar does not evaluate its first argument. Each time the interpreter encounters a symbol, it first checks to see if that symbol is a special form.
- Now that A-4 is defined, it can be freely used in other expressions:
(defvar A-5 (* A-4 2))
- The defparameter macro is a twin of defvar, it is also used to define and bind new global variables (symbols).
Preventing and forcing Evaluation.
- Suppose you want to bind the symbol F-maj to the list (F-4 A-4 C-5). You might tempted to try the following:
(defvar F-maj (F-4 A-4 C-5))
This produces an error because the interpreter is trying to evaluate the list (F-4 A-4 C-5) as a function call, interpreting the symbol F-4 as a function and the rest as its arguments. We have to tell the interpreter to not evaluate this list and to assign it as the value for the symbol F-maj.
- We can prevent evaluation using the special form quote to wrap the expressions that are not to be evaluated with it:
(defvar F-maj (quote (F-4 A-4 C-5)))
- As quote is used so frequently in Lisp there is a shorthand for it; this involves prefixing the expression to be "quoted" with the single-quote character. The following expression is equivalent to the previous one:
(defvar F-maj '(F-4 A-4 C-5))
- Now, suppose we want our chord to be one octave lower:
(defvar F-maj '(F-3 A-3 C-4))
- Note that defvar has no effect if the variable represented by its first argument has already been defined:
- The setf macro lets us change a symbol's binding:
(setf F-maj '(F-3 A-3 C-4))
Note that setf evaluates to its second argument. Now the symbol F-maj is bound to its new value:
- The setf macro can be used to instantiate multiple bindings of symbols and values:
(setf F-3 174.61 A-3 (/ A-4 2) C-4 261.62 A-1 (/ A-3 2))
Note that you can use setf to create new variables too.
- Odd-numbered arguments are not evaluated and even-numbered arguments are evaluated; the odd-numbered arguments represent the symbols that are to be bound to the even-numbered arguments. The way setf evaluates its arguments is called sequential fashion, that's why we can use the in-line defined variable A-3 to define A-1.
- The special form setq does the same work as setf. It is mostly use as a simple variable assignment statement in contrast to setf that is used as a general assignment statement. This difference is a matter of style: to change the value of a global variables setf is used and for simple or local variables setq is used.
- The function set behaves the as setf and setq but it evaluates its first argument.
Portions of this page come from Todd Feldman's handouts for CS 193L.
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©1996-98 by Juan Pampin, email@example.com