EVAL Notation

@函数式编程初探 in EverNote-Lisp

In Lisp,functions are data

EVALNOTATION CAN DO ANYTHING BOX NOTATION CAN DO

Evaluation Rule for Numbers, T, and NIL: Numbers, and the symbols T and NIL, evaluate to themselves.
Evaluation Rulefor Lists: The first element of the list specifies a function to be called. The remaining elementsspecify arguments to the function. Thefunction is called on the evaluated arguments.
Evaluation Rule for Symbols: A symbol evaluates to the value of the variable it refers to.

macro function 宏函数

(defun funciont_name (argument list) (body))

(defun average (x y)
(/ (+ x y) 2.0))

Variables are not symbols; variables are named by symbols. Functionsare also named by symbols.

unassigned variable error (defun riddle (x y) (list ’why ’is ’a x ’like ’a y))

(riddle ’raven ’writing-desk) ⇒
(why is a raven like a writing-desk)

Lists also need to be quoted to use them as data

(first (we hold these truths))
⇒ Error! WE undefined function.
(first ’(we hold these truths)) ⇒ we

example

(+ 1 2) ⇒ 3
’(+ 1 2) ⇒ (+ 1 2)
(oddp (+ 1 2)) ⇒ t
(oddp ’(+ 1 2)) ⇒ Error! Wrong type input to ODDP.

’(foo bar baz) ⇒ (foo bar baz)
(list ’foo ’bar ’baz) ⇒ (foo bar baz)
(cons ’foo ’(bar baz)) ⇒ (foo bar baz)

old style quote = '

’’foo ⇒ ’foo also written (quote foo)
(list ’quote ’foo) ⇒ (quote foo) also written ’foo
(first ’’foo) ⇒ quote

(EQUAL 3 5)

LAMBDA is not a function;it is a marker treated specially by EVAL.

lambda is the symbol for an anonymous function

(lambda (x y) (+ (* 3 x) (* y y)))

It is possible to write a function whose value is another function

(defun make-greater-than-predicate (n)
#’(lambda (x) (> x n)))