# etags - Build a TAG for Multiple R Packages

Here is what tried to build a TAG for multiple R packages. It enable me to jump to a location where the function/variable is defined and modify if I want to.

Useful variable and functions

ess-r-package-library-path
default path to find packages, should be a list
ess-r-package-root-file
if the folder has DESCRIPTION file, then the folder is a R package.
(ess-build-tags-for-directory DIR TAGFILE)
build tag on DIR to TARGET.
tags-table-list
List of file names of tags tables to search.
(visit-tags-table FILE &optional LOCAL)
Tell tags commands to use tags table file.

Note the workhorse is ess-build-tags-for-directory which does what it means. The core of this function use find and etags program. The find program will find files with extension .cpp, R, nw etc, and then feed to (using pipe) to the etags program which generate a TAG table. These two steps are demonstrated in the following snippet, which is grabbed from the source code of ess-build-tags-for-directory.

Note when they are used in Emacs, the tags-table-list variable is appended with the path to the new TAG table. So that the user can use xref-find-definitions (M-.) to jump (if the point is under a word) or select which function/variable to jump to. The users then check the function/variable definition, or modify it if it is necessary. Then call xref-pop-marker-stack (M-,) to jump back.

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