R: Experimental Alternative Name Specification Support

ns-alt {base} | R Documentation

## Experimental Alternative Name Specification Support

### Description

Alternative interface for specifying a name space within the code of a package.

### Usage

    .Export(...)
    .Import(...)
    .ImportFrom(name, ...)
    .S3method(generic, class, method)

### Arguments

`...` | name or literal character string arguments.
`name` | name or literal character string.
`generic` | name or literal character string.
`class` | name or literal character string.
`method` | optional character or function argument.

### Details

As an experimental alternative to using a ‘NAMESPACE’ file it is possible to add a name space to a package by adding a `Namespace: <package_name>` entry to the ‘DESCRIPTION’ file and placing directives to specify imports and exports directly in package code. These directives should be viewed as declarations, not as function calls. Except to the optional method argument to `.S3method` arguments are not evaluated. These directives should only be used at top level of package code except as noted below.

`.Export` is used to declare exports. Its arguments should be literal names or character strings. `.Export` should only be used at package top level.

`.Import` is used to declare the import of entire name spaces. Its arguments should be literal names or character strings. `.ImportFrom` is used to declare the import of selected variables from a single name space. The first argument is a literal name or character string identifying the source name space; the remaining arguments are literal names or character strings identifying the variables to import. As an experimental feature both `.Import` and `.ImportFrom` can be used to import variables into a local environment. The drawback of allowing this is that dependencies cannot be determined easily at package load time, and as a result this feature may need to be dropped.

`.S3method` is used to declare a method for S3-style `UseMethod` dispatch. This is needed since methods in packages that are imported but not on the search path might not be visible to the standard dispatch mechanism at a call site. The first argument is the name of the generic, the second specifies the class. The third argument is optional and defaults to the usual concatenation of generic and class separated by a period. If supplied, the third argument should evaluate to a character string or a function. If the third argument is omitted or a character string is supplied, then a function by that name must be defined. If a function is supplied, it is used as the method. When the method is specified as a name, explicitly or implicitly, the function lookup is handled lazily; this allows the definition to occur after the `.S3method` declaration and also integrates with possible data base storage of package code.

### Author(s)

Luke Tierney

### Examples

    ## Not run:
    ## code for package/name space 'foo'
    x <- 1
    f <- function(y) c(x,y)
    print.foo <- function(x, ...) cat("<a foo>\n")
    .Export(f)
    S3method(print,foo)

    ## code for package/name space 'bar'
    .Import(foo)
    c <- function(...) sum(...)
    g <- function(y) f(c(y, 7))
    h <- function(y) y+9
    .Export(g, h)
    ## End(Not run)

[Package base version 2.8.1 Index]