SLSH Library Reference (version 2.2.0): The require Function

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## 2. The `require` Function

The functions in `require.sl` facilitate the loading of S-Lang code.

## 2.1 _featurep

Synopsis

  Test whether or not a feature is present

Usage

  `Int_Type _featurep (String_Type feature [,String_Type namespace])`

Description

  The `_featurep` function returns a non-zero value if the specified feature is present. Otherwise, it returns 0 to indicate that the feature has not been loaded.

See Also

  `require, provide`

## 2.2 provide

Synopsis

  Declare that a specified feature is available

Usage

  `provide (String_Type feature [,String_Type namespace])`

Description

  The `provide` function may be used to declare that a "feature" has been loaded into the specified namespace. If the namespace argument is not present, the current namespace will be used. See the documentation for `require` for more information.

See Also

  `require, _featurep`

## 2.3 require

Synopsis

  Make sure a feature is present, and load it if not

Usage

  `require (feature [,namespace [,file]])`

  > `

  >        String_Type feature, namespace, file;

  > `

Description

  The `require` function ensures that a specified "feature" is present. If the feature is not present, the `require` function will attempt to load the feature from a file. If the `namespace` argument is present and non-NULL, the specified namespace will be used. The default is to use the current non-anonymous namespace. If called with three arguments, the feature will be loaded from the file specified by the third argument if it does not already exist in the namespace. Otherwise, the feature will be loaded from a file given by the name of the feature, with ".sl" appended.

  If after loading the file, if the feature is not present, a warning message will be issued.

Example

  > `

  >         require ("histogram");
  >         require ("histogram", "foo");
  >         require ("histogram", "foo", "/home/bob/hist.sl");
  >         require ("histogram", ,"/home/bob/hist.sl");

  > `

Notes

  "feature" is an abstract quantity that is undefined here.

  A popular use of the `require` function is to ensure that a specified file has already been loaded. In this case, the feature is the filename itself. The advantage of using this mechanism over using `evalfile` is that if the file has already been loaded, `require` will not re-load it.

See Also

  `provide, _featurep, evalfile`

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