CS61A Lecture 18. Amir Kamil UC Berkeley March 4, 2013

Transcription

1 CS61A Lecture 18 Amir Kamil UC Berkeley March 4, 2013

2 Announcements HW6 due on Thursday Trends project due tomorrow Ants project out

3 Persistent Local State The parent contains local state A function with a parent frame Every call changes the balance Example:

4 Non Local Assignment def make_withdraw(balance): """Return a withdraw function with a starting balance.""" def withdraw(amount): nonlocal balance Declare the name "balance" nonlocal if amount > balance: return 'Insufficient funds' balance = balance - amount return balance return withdraw Re bind balance where it was bound previously

5 Mutable Values and Persistent State

6 Mutable Values and Persistent State Mutable values can be changed without a nonlocal statement.

7 Mutable Values and Persistent State Mutable values can be changed without a nonlocal statement. Example:

8 Mutable Values and Persistent State Mutable values can be changed without a nonlocal statement. Name value binding cannot change Example:

9 Mutable Values and Persistent State Mutable values can be changed without a nonlocal statement. Mutable value can change Name value binding cannot change Example:

10 Creating Two Withdraw Functions Example:

11 Multiple References to a Withdraw Function Example:

12 The Benefits of Non Local Assignment

13 The Benefits of Non Local Assignment Ability to maintain some state that is local to a function, but evolves over successive calls to that function.

14 The Benefits of Non Local Assignment Ability to maintain some state that is local to a function, but evolves over successive calls to that function. The binding for balance in the first non local frame of the environment associated with an instance of withdraw is inaccessible to the rest of the program.

15 The Benefits of Non Local Assignment Ability to maintain some state that is local to a function, but evolves over successive calls to that function. The binding for balance in the first non local frame of the environment associated with an instance of withdraw is inaccessible to the rest of the program. An abstraction of a bank account that manages its own internal state.

16 The Benefits of Non Local Assignment Ability to maintain some state that is local to a function, but evolves over successive calls to that function. The binding for balance in the first non local frame of the environment associated with an instance of withdraw is inaccessible to the rest of the program. An abstraction of a bank account that manages its own internal state. Weasley Account $10

17 The Benefits of Non Local Assignment Ability to maintain some state that is local to a function, but evolves over successive calls to that function. The binding for balance in the first non local frame of the environment associated with an instance of withdraw is inaccessible to the rest of the program. An abstraction of a bank account that manages its own internal state. Weasley Account $10 Potter Account $1,000,000

18 Referential Transparency

19 Referential Transparency Expressions are referentially transparent if substituting an expression with its value does not change the meaning of a program.

20 Referential Transparency Expressions are referentially transparent if substituting an expression with its value does not change the meaning of a program. mul(add(2, mul(4, 6)), 3)

21 Referential Transparency Expressions are referentially transparent if substituting an expression with its value does not change the meaning of a program. mul(add(2, mul(4, 6)), 3) mul(add(2, 24 ), 3)

22 Referential Transparency Expressions are referentially transparent if substituting an expression with its value does not change the meaning of a program. mul(add(2, mul(4, 6)), 3) mul(add(2, 24 ), 3) mul( 26, 3)

23 Referential Transparency Expressions are referentially transparent if substituting an expression with its value does not change the meaning of a program. mul(add(2, mul(4, 6)), 3) mul(add(2, 24 ), 3) mul( 26, 3) Mutation is a side effect (like printing)

24 Referential Transparency Expressions are referentially transparent if substituting an expression with its value does not change the meaning of a program. mul(add(2, mul(4, 6)), 3) mul(add(2, 24 ), 3) mul( 26, 3) Mutation is a side effect (like printing) Side effects violate the condition of referential transparency because they do more than just return a value; they change the state of the computer.

25 Referential Transparency Expressions are referentially transparent if substituting an expression with its value does not change the meaning of a program. mul(add(2, mul(4, 6)), 3) mul(add(2, 24 ), 3) mul( 26, 3) Mutation is a side effect (like printing) Side effects violate the condition of referential transparency because they do more than just return a value; they change the state of the computer.

26 Referential Transparency Expressions are referentially transparent if substituting an expression with its value does not change the meaning of a program. mul(add(2, mul(4, 6)), 3) mul(add(2, 24 ), 3) mul( 26, 3) Mutation is a side effect (like printing) Side effects violate the condition of referential transparency because they do more than just return a value; they change the state of the computer.

27 A Mutable Container

28 A Mutable Container def container(contents):

29 A Mutable Container def container(contents): """Return a container that is manipulated by two functions.

30 A Mutable Container def container(contents): """Return a container that is manipulated by two functions. >>> get, put = container('hello')

31 A Mutable Container def container(contents): """Return a container that is manipulated by two functions. >>> get, put = container('hello') >>> get()

32 A Mutable Container def container(contents): """Return a container that is manipulated by two functions. >>> get, put = container('hello') >>> get() 'hello'

33 A Mutable Container def container(contents): """Return a container that is manipulated by two functions. >>> get, put = container('hello') >>> get() 'hello' >>> put('world')

34 A Mutable Container def container(contents): """Return a container that is manipulated by two functions. >>> get, put = container('hello') >>> get() 'hello' >>> put('world') >>> get()

35 A Mutable Container def container(contents): """Return a container that is manipulated by two functions. >>> get, put = container('hello') >>> get() 'hello' >>> put('world') >>> get() 'world'

36 A Mutable Container def container(contents): """Return a container that is manipulated by two functions. >>> get, put = container('hello') >>> get() 'hello' >>> put('world') >>> get() 'world' """

37 A Mutable Container def container(contents): """Return a container that is manipulated by two functions. >>> get, put = container('hello') >>> get() 'hello' >>> put('world') >>> get() 'world' """ def get():

38 A Mutable Container def container(contents): """Return a container that is manipulated by two functions. >>> get, put = container('hello') >>> get() 'hello' >>> put('world') >>> get() 'world' """ def get(): return contents

39 A Mutable Container def container(contents): """Return a container that is manipulated by two functions. >>> get, put = container('hello') >>> get() 'hello' >>> put('world') >>> get() 'world' """ def get(): return contents def put(value):

40 A Mutable Container def container(contents): """Return a container that is manipulated by two functions. >>> get, put = container('hello') >>> get() 'hello' >>> put('world') >>> get() 'world' """ def get(): return contents def put(value): nonlocal contents

41 A Mutable Container def container(contents): """Return a container that is manipulated by two functions. >>> get, put = container('hello') >>> get() 'hello' >>> put('world') >>> get() 'world' """ def get(): return contents def put(value): nonlocal contents contents = value

42 A Mutable Container def container(contents): """Return a container that is manipulated by two functions. >>> get, put = container('hello') >>> get() 'hello' >>> put('world') >>> get() 'world' """ def get(): return contents def put(value): nonlocal contents contents = value return put, get

43 Dispatch Functions

44 Dispatch Functions A technique for packing multiple behaviors into one function

45 Dispatch Functions A technique for packing multiple behaviors into one function def pair(x, y): """Return a function that behaves like a pair.""" def dispatch(m): if m == 0: return x elif m == 1: return y return dispatch

46 Dispatch Functions A technique for packing multiple behaviors into one function def pair(x, y): """Return a function that behaves like a pair.""" def dispatch(m): if m == 0: return x elif m == 1: return y return dispatch Message argument can be anything, but strings are most common

47 Dispatch Functions A technique for packing multiple behaviors into one function def pair(x, y): """Return a function that behaves like a pair.""" def dispatch(m): if m == 0: return x elif m == 1: return y return dispatch Message argument can be anything, but strings are most common The body of a dispatch function is always the same: st common

48 Dispatch Functions A technique for packing multiple behaviors into one function def pair(x, y): """Return a function that behaves like a pair.""" def dispatch(m): if m == 0: return x elif m == 1: return y return dispatch Message argument can be anything, but strings are most common The body of a dispatch function is always the same: One conditional statement with several clauses

49 Dispatch Functions A technique for packing multiple behaviors into one function def pair(x, y): """Return a function that behaves like a pair.""" def dispatch(m): if m == 0: return x elif m == 1: return y return dispatch Message argument can be anything, but strings are most common The body of a dispatch function is always the same: One conditional statement with several clauses Headers perform equality tests on the message

50 Message Passing

51 Message Passing An approach to organizing the relationship among different pieces of a program

52 Message Passing An approach to organizing the relationship among different pieces of a program Different objects pass messages to each other

53 Message Passing An approach to organizing the relationship among different pieces of a program Different objects pass messages to each other What is your fourth element?

54 Message Passing An approach to organizing the relationship among different pieces of a program Different objects pass messages to each other What is your fourth element? Change your third element to this new value. (please?)

55 Message Passing An approach to organizing the relationship among different pieces of a program Different objects pass messages to each other What is your fourth element? Change your third element to this new value. (please?) Encapsulates the behavior of all operations on a piece of data

56 Message Passing An approach to organizing the relationship among different pieces of a program Different objects pass messages to each other What is your fourth element? Change your third element to this new value. (please?) Encapsulates the behavior of all operations on a piece of data Important historical role: The message passing approach strongly influenced object oriented programming (next lecture)

57 Mutable Container with Message Passing def container(contents): def get(): return contents def put(value): nonlocal contents contents = value return put, get

58 Mutable Container with Message Passing def container_dispatch(contents): def container(contents): def get(): return contents def put(value): nonlocal contents contents = value return put, get

59 Mutable Container with Message Passing def container_dispatch(contents): def container(contents): def dispatch(message, value=none): def get(): return contents def put(value): nonlocal contents contents = value return put, get

60 Mutable Container with Message Passing def container_dispatch(contents): def container(contents): def dispatch(message, value=none): nonlocal contents def get(): return contents def put(value): nonlocal contents contents = value return put, get

61 Mutable Container with Message Passing def container_dispatch(contents): def container(contents): def dispatch(message, value=none): nonlocal contents if message == 'get': def get(): return contents def put(value): nonlocal contents contents = value return put, get

62 Mutable Container with Message Passing def container_dispatch(contents): def container(contents): def dispatch(message, value=none): nonlocal contents if message == 'get': return contents def get(): return contents def put(value): nonlocal contents contents = value return put, get

63 Mutable Container with Message Passing def container_dispatch(contents): def container(contents): def dispatch(message, value=none): nonlocal contents if message == 'get': return contents if message == put': def get(): return contents def put(value): nonlocal contents contents = value return put, get

64 Mutable Container with Message Passing def container_dispatch(contents): def container(contents): def dispatch(message, value=none): nonlocal contents if message == 'get': return contents if message == put': contents = value def get(): return contents def put(value): nonlocal contents contents = value return put, get

65 Mutable Container with Message Passing def container_dispatch(contents): def container(contents): def dispatch(message, value=none): nonlocal contents if message == 'get': return contents if message == put': contents = value def get(): return contents def put(value): nonlocal contents contents = value return dispatch return put, get

66 Mutable Container with Message Passing def container_dispatch(contents): def container(contents): def dispatch(message, value=none): nonlocal contents if message == 'get': return contents if message == put': contents = value def get(): return contents def put(value): nonlocal contents contents = value return dispatch return put, get

67 Mutable Container with Message Passing def container_dispatch(contents): def container(contents): def dispatch(message, value=none): nonlocal contents if message == 'get': return contents if message == put': contents = value def get(): return contents def put(value): nonlocal contents contents = value return dispatch return put, get

68 Mutable Recursive Lists

69 Mutable Recursive Lists def mutable_rlist():

70 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist

71 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none):

72 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none): nonlocal contents

73 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none): nonlocal contents if message == 'len':

74 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none): nonlocal contents if message == 'len': return len_rlist(contents)

75 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none): nonlocal contents if message == 'len': return len_rlist(contents) elif message == 'getitem':

76 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none): nonlocal contents if message == 'len': return len_rlist(contents) elif message == 'getitem': return getitem_rlist(contents, value)

77 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none): nonlocal contents if message == 'len': return len_rlist(contents) elif message == 'getitem': return getitem_rlist(contents, value) elif message == 'push':

78 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none): nonlocal contents if message == 'len': return len_rlist(contents) elif message == 'getitem': return getitem_rlist(contents, value) elif message == 'push': contents = make_rlist(value, contents)

79 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none): nonlocal contents if message == 'len': return len_rlist(contents) elif message == 'getitem': return getitem_rlist(contents, value) elif message == 'push': contents = make_rlist(value, contents) elif message == 'pop':

80 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none): nonlocal contents if message == 'len': return len_rlist(contents) elif message == 'getitem': return getitem_rlist(contents, value) elif message == 'push': contents = make_rlist(value, contents) elif message == 'pop': item = first(contents)

81 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none): nonlocal contents if message == 'len': return len_rlist(contents) elif message == 'getitem': return getitem_rlist(contents, value) elif message == 'push': contents = make_rlist(value, contents) elif message == 'pop': item = first(contents) contents = rest(contents)

82 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none): nonlocal contents if message == 'len': return len_rlist(contents) elif message == 'getitem': return getitem_rlist(contents, value) elif message == 'push': contents = make_rlist(value, contents) elif message == 'pop': item = first(contents) contents = rest(contents) return item

83 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none): nonlocal contents if message == 'len': return len_rlist(contents) elif message == 'getitem': return getitem_rlist(contents, value) elif message == 'push': contents = make_rlist(value, contents) elif message == 'pop': item = first(contents) contents = rest(contents) return item elif message == 'str':

84 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none): nonlocal contents if message == 'len': return len_rlist(contents) elif message == 'getitem': return getitem_rlist(contents, value) elif message == 'push': contents = make_rlist(value, contents) elif message == 'pop': item = first(contents) contents = rest(contents) return item elif message == 'str': return str_rlist(contents)

85 Mutable Recursive Lists def mutable_rlist(): contents = empty_rlist def dispatch(message, value=none): nonlocal contents if message == 'len': return len_rlist(contents) elif message == 'getitem': return getitem_rlist(contents, value) elif message == 'push': contents = make_rlist(value, contents) elif message == 'pop': item = first(contents) contents = rest(contents) return item elif message == 'str': return str_rlist(contents) return dispatch