Transcript Lisp S-Expressions: ATOMs

Lisp S-Expressions: ATOMs
Every Lisp object is either an ATOM or a CONS
Symbols and numbers are kinds of atoms:
X, APPLE, A-SYMBOL
1, 5.7, 3/5
Many other Lisp data objects are considered to be
atoms (even strings and arrays are atoms!).
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Lisp S-Expressions: CONSes
Every Lisp object is either an ATOM or a CONS
A CONS represents an association or pairing of two
other Lisp objects.
(A . B)
(APPLE . RED)
(PI . 3.14159)
(X . (Y . Z))
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Lisp S-Expressions: Lists
We define lists as follows:
The symbol NIL is a list; it’s the empty list.
This list is written in list notation as ( )
Any cons having the following structure is a
list,
(S1 . (S2 . ( ... (Sn . NIL) ... ) ) )
where each S1 is either an atom or a cons.
This list is written in list notation as
(S1 S2 ... Sn)
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Examples of Lists
> ’(a b c d e)
(A B C D E)
> ()
NIL
> nil
NIL
> ’()
NIL
> ’(apple . (banana . (lime . nil)))
(APPLE BANANA LIME)
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Predicates That Identify Lists
> (atom ’(a b c))
NIL
> (atom ’x)
T
> (consp ’(a b c))
T
> (consp ’x)
NIL
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List predicates (continued)
> (listp
T
> (listp
NIL
> (consp
NIL
> (listp
T
> (consp
T
> (listp
T
’(a b c))
’x)
’()) ; NIL is not a cons.
’()) ; NIL is a list.
’(a . b))
’(a . b)) ;note listp’s limitation.
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Lisp Tries to Print Conses as Lists
> ’(a . (b . c))
(A B . C)
> ’(a . nil)
(A)
> ’((a . b) . (c . d))
((A . B)C . D)
> ’((nil . nil) . (nil . nil))
((NIL)NIL)
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Lisp Forms
A form is a list whose first element is a symbol
that names an operator.
If the first element names a function, then the
form is a functional form.
> (+ 1 2 3) ; a functional form
6
> (functionp #’+)
T
> (setq x 5) ; a special form
5
> (functionp #’setq) ;
Error!
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Evaluation of Functional Forms
A functional form is evaluted as follows:
If there are any arguments in the form, then they
are evaluated in left-to-right order.
The number of arguments is compared with the
number permitted by the function named by the
first element of the form.
If the number is compatible, then the function is
applied to the values of the arguments.
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QUOTE
Unlike functional forms, special forms are not
required to have their arguments evaluated.
QUOTE is a special form that returns its
argument unevaluated.
> (quote (+ 1 2 3))
(+ 1 2 3)
> (quote x)
X
> ’(+ 1 2 3)
(+ 1 2 3)
> ’x
X
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SETQ
SETQ is a special form that evaluates its second
argument and assigns that value to the symbol
which must be its first argument.
> (setq x
6
> (setq x
18
> (setq y
(+ 1 2 3)
> (setq y
(1 2 3)
> (setq y
1
(+ 1 2 3))
(* x 3))
’(+ 1 2 3))
(rest y))
(first y))
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IF
IF is a special form that evaluates its first
argument. If the result is NIL is skips its second
argument and evaluates and returns its their
element if any. If result of evaluating the first
element was not NIL, it evaluates the second
argument and returns that.
> (setq x 10)
10
> (if (> x 2) (- x 1) (+ x 1))
9
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