1\function{_featurep}
2\synopsis{Test whether or not a feature is present}
3\usage{Int_Type _featurep (String_Type feature [,String_Type namespace])}
4\description
5  The \sfun{_featurep} function returns a non-zero value if the specified
6  feature is present.  Otherwise, it returns 0 to indicate that the feature
7  has not been loaded.
8\seealso{require, provide}
9\done
10
11\function{provide}
12\synopsis{Declare that a specified feature is available}
13\usage{provide (String_Type feature [,String_Type namespace])}
14\description
15 The \sfun{provide} function may be used to declare that a "feature" has
16 been loaded into the specified namespace.  If the namespace argument is not
17 present, the current namespace will be used.
18 See the documentation for \sfun{require} for more information.
19\seealso{require, _featurep}
20\done
21
22\function{require}
23\synopsis{Make sure a feature is present, and load it if not}
24\usage{require (feature [,namespace [,file]])}
25#v+
26   String_Type feature, namespace, file;
27#v-
28\description
29  The \sfun{require} function ensures that a specified "feature" is present.
30  If the feature is not present, the \sfun{require} function will attempt to
31  load the feature from a file.  If the \exmp{namespace} argument is present
32  and non-NULL, the specified namespace will be used.  The default is to use
33  the current non-anonymous namespace. If called with three arguments, the
34  feature will be loaded from the file specified by the third argument
35  if it does not already exist in the namespace.  Otherwise, the feature
36  will be loaded from a file given by the name of the feature, with
37  ".sl" appended.
38
39  If after loading the file, if the feature is not present,
40  a warning message will be issued.
41\example
42#v+
43    require ("histogram");
44    require ("histogram", "foo");
45    require ("histogram", "foo", "/home/bob/hist.sl");
46    require ("histogram", ,"/home/bob/hist.sl");
47#v-
48\notes
49  "feature" is an abstract quantity that is undefined here.
50
51  A popular use of the \sfun{require} function is to ensure that a specified
52  file has already been loaded.  In this case, the feature is the
53  filename itself.  The advantage of using this mechanism over using
54  \ifun{evalfile} is that if the file has already been loaded, \sfun{require}
55  will not re-load it.
56\seealso{provide, _featurep, evalfile}
57\done
58