1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
|
=================================================================
Logtalk - Object oriented extension to Prolog
Release 2.27.1
Copyright (c) 1998-2006 Paulo Moura. All Rights Reserved.
=================================================================
% start by loading the example:
| ?- logtalk_load(viewpoints(loader)).
...
% we can start by asking joe its age:
| ?- joePerson::age(Age).
Age = 30
yes
% the same question could be made via any of its viewpoints:
| ?- joeSportsman::age(Age).
Age = 30
yes
% now let's tell joe to get older:
| ?- joePerson::getOlder.
yes
% we can verify the effect of the above message from any of the viewpoints:
| ?- joeChessPlayer::age(Age).
Age = 31
yes
% because the growOld/0 and the age/1 predicates are implemented using
% property sharing, we can send the getOlder/0 message to any viewpoint:
| ?- joeEmployee::getOlder.
yes
% we can check this by asking joe its age:
| ?- joePerson::age(Age).
Age = 32
yes
% as you can see, although the modification message have been sent to a
% descendant, its the predicate age/1 in the parent that got updated
% to illustrate value sharing we use a couple of predicates, score/1 and
% setScore/0, defined in joePerson:
| ?- joePerson::score(Score).
Score = 0
yes
% initially, score/1 is only defined for joePerson, so every descendant
% or viewpoint will share its value/definition:
| ?- joeEmployee::score(Score).
Score = 0
yes
% but if we decide to increment the counter by sending the setScore/0
% message to a descendant:
| ?- joeChessPlayer::(setScore(2200), score(Score)).
Score = 2200
yes
% then the descendant will now have a local definition for counter/1,
% independent of the definition in its parent, joePerson:
| ?- joePerson::score(Score).
Score = 0
yes
% the other descendants/viewpoints will continue to share the definition
% in joePerson:
| ?- joeSportsman::score(Score).
Score = 0
yes
|
