-
Notifications
You must be signed in to change notification settings - Fork 2
/
Copy pathmethodsPackage.html
98 lines (95 loc) · 4.82 KB
/
methodsPackage.html
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
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
<html> <head>
<link REL=stylesheet HREF=Rtech.css>
<title>The `methods' package in the r-devel code</title>
</head>
<body>
<h1>The <code>methods</code> package in the r-devel code</h1>
A package, <code>methods</code>, has been added to the source
repository for the 1.4 version of R.
<p>
The contents provide an implementation of formal classes and methods,
treating the description in the book <i>Programming with Data</i>
(aka the green book). The following notes introduce a
few topics for discussion regarding R development. (The source of the
package replaces, and updates, the Omegahat module
<code>R/Classes</code>.)
<ol>
<li> <i>Where should the code reside?</i><p>
The code is currently being kept in a package, in the main CVS tree so it's
guaranteed to be shipped with the corresponding development
version of R.<p>
This may be a temporary situation, but it has the advantage that one
can detach the package, if it turns out to break something
else (e.g., see the next list item). Discussions so far suggest
that in the long run at least part of the code will need to be
pretty tightly bound to the evaluator for efficiency and to work
well with some other new developments such as threading.<p>
The eventual choice may be to incorporate all of the package
into the base. We should discuss this as things move along.
<p>
<li> <i>Revised versions of <code>class</code>, etc.</i><p>
You will see when attaching <code>methods</code> that it
overrides <code>class</code>. This is a potential source of
non-compatibility with old-style class code. Details are in a
<a href="classIssues.html">separate note</a>.<p>
<li> <i>Extensions to the Grammar</i><p>
The <code>@</code> operator has been added for accessing
slots--this is pretty trivial, but as it's implemented as a
primitive, it is now in the base.<p>
I would also like to add a <i>limited</i> use of
<code>=</code> for assignments: only to be allowed for
top-level assignments or for elements of a braced list. The
limitation removes any chance of the notorious
<pre><code>
if(x = 0) /* do something important */
</code></pre>
C bug, but still allows the PwD examples to be run. (It's also
useful, I think, in bringing in new users who have programmed in
C, Java, even Fortran.)<p>
<li> <i>S3 Classes and Methods</i><p>
We need eventually to decide how to deal with these, if it turns
out we want to make S4-style methods the standard. The current
implementation is fairly back-compatible, in the sense for
example that one can treat matrices as a formal class and
usually find that existing matrices conform. (Because R slots
are implemented as attributes.)<p>
Some points for discussion are collected in a <a
href="classIssues.html">separate page</a>.<p>
<li> <i>Inconsistencies with the Book</i><p><a name="Inconsistencies"></a>
The following are some of the main known differences from the
description in <i>Programming with Data</i>.
<ul>
<li> The functions explicitly dealing with ``old-style''
classes are not implemented. They probably will be,
although the R developers need to discuss the long-term
strategy for the two class models (see some <a href="classIssues.html">discussion points</a>).
<li> The concept of meta-databases to store the definitions of
classes and methods has not been carried over to R.
Instead, a combination of the use of function environments
(for generic functions) and name-mangling (for classes)
stores the information. The official programming model
almost never deals with the metadata explicitly, in either
case.
<li> Documentation of classes and methods and special
mechanisms to access it (such as <code>?f(x, y)</code> or
<code>class?myClass</code>). Suggestions would be welcome
on how to do this. Also, decisions about the future of
documentation in R, generally, will make a difference on
how we should deal with special documentation for classes
and methods.
<li> Other specific tools do not yet exist, but mostly just
from lack of time or occasionally some question about the
best implementation. Examples include:
tools (like <code>showMethods</code>) that work across
multiple generic functions; validity methods for classes;
version management (which has never developed much for
S-Plus either).
</ul>
</ol>
<hr>
<address><a href="http://cm.bell-labs.com/cm/ms/departments/sia/jmc/">John Chambers</a><a href=mailto:[email protected]><[email protected]></a></address>
<!-- hhmts start -->
Last modified: Mon Aug 13 14:59:33 EDT 2001
<!-- hhmts end -->
</body> </html>