Scheme programs consist of expressions( everything is expressions ), which can be:
•Primitive expressions: 2, 3.3, true, +, quotient, ...
•Combinations: (quotient 10 2), (not true), ...
1. Primative expressions
Numbers are self-evaluating; symbols(just like name in Python) are bound to values
scm> 123 123 scm> 123.123 123.123 scm> #t #t scm> #f #f scm> "asdf" "asdf" scm> ’a ; this is a symbol a
when you use the single quote, you’re telling Scheme not to follow the normal rules of evaluation and just have the symbol return as itself.
2. Function Calls
Call expressions include an operator and 0 or more operands in parentheses
scm> (+ 1 2) 3 scm> (- 2 3) -1 scm> (* 6 3) 18 scm> (/ 5 2) 2.5 scm> (+ 1 (* 3 4)) 13
some userful functions
+, -, *, / • eq?, =, >, >=, <, <=
3. Special forms
A combination that is not a call expression is a special form:
• if expression: (if <condition> <then> <else>)
• and or not: (and <e1> ... <en>), (or <e1> ... <en>), (not <en>)
• Binding symbols: (define <symbol> <expression>)
• New procedures: (define (<symbol> <formal parameters>) <body>)
scm> (if ’this-evaluates-to-true 1 2) 1 scm> (and 1 2 3) 3 scm> (or 1 2 3) 1
Defining functions
scm> (define (square x) (* x x)) square scm> (square 5) 25
Lambda Expressions
Lambda expressions evaluate to anonymous procedures
(lambda (<formal-parameters>) <body>)
Two equivalent expressions:
(define (plus4 x) (+ x 4)) (define plus4 (lambda (x) (+ x 4)))
Ps:When you do (define ( ) ), Scheme will automatically transform it to (define (lambda () ). In this way, lambdas are more central to Scheme than they are to Python.
4. Compound values
Pairs and Lists
• cons: Two-argument procedure that creates a pair
• car: Procedure that returns the first element of a pair
• cdr: Procedure that returns the second element of a pair
• nil: The empty list
Pairs
> (define x (cons 1 2)) > x (1 . 2) #Dots can be used in a quoted list to specify the second element of the final pair. > (car x) 1 > (cdr x)Dots can be used in a quoted list to specify the second element of the final pair. 2 > (cons 1 (cons 2 (cons 3 (cons 4 nil)))) (1 2 3 4)
List
scm> ’() () scm> nil () scm> (cons 1 (cons 2 nil)) (1 2) scm> (cons 1 (cons 2 (cons 3 nil))) (1 2 3) scm> ’(1 2 3) (1 2 3)
5. Symbolic Programming
Symbols normally refer to values
> (define a 1) > (define b 2) > (list a b) (1 2)
Quotation is used to refer to symbols directly in Lisp.
> (list ‘a ‘b) (a b) > (list ‘a b) (a 2)
Quotation can also be applied to combinations to form lists.
> (car ‘(a b c)) a > (cdr ‘(a b c)) (b c)
Ps:scheme 中没有while, for等, 所有要用到迭代的地方都用递归
2015-07-15