#+TITLE: Helm Gtags layer

* Table of Contents                                          :TOC_4_gh:noexport:
 - [[#description][Description]]
 - [[#features][Features]]
 - [[#install][Install]]
   - [[#gnu-global-gtags][GNU Global (gtags)]]
     - [[#install-on-osx-using-homebrew][Install on OSX using Homebrew]]
     - [[#install-on-nix-from-source][Install on *nix from source]]
       - [[#install-recommended-dependencies][Install recommended dependencies]]
       - [[#install-with-recommended-features][Install with recommended features]]
       - [[#configure-your-environment-to-use-pygments-and-ctags][Configure your environment to use pygments and ctags]]
   - [[#emacs-configuration][Emacs Configuration]]
     - [[#disabling-by-default][Disabling by default]]
 - [[#usage][Usage]]
     - [[#language-support][Language Support]]
       - [[#built-in-languages][Built-in languages]]
       - [[#exuberant-ctags-languages][Exuberant ctags languages]]
       - [[#universal-ctags-languages][Universal ctags languages]]
       - [[#pygments-languages-plus-symbol-and-reference-tags][Pygments languages (plus symbol and reference tags)]]
   - [[#eldoc-integration][Eldoc integration]]
 - [[#key-bindings][Key bindings]]

* Description
=helm-gtags= and =ggtags= are clients for GNU Global. GNU Global is a source
code tagging system that allows querying symbol locations in source code, such
as definitions or references. Adding the =gtags= layer enables both of these
modes.

* Features

- Select any tag in a project retrieved by gtags
- Resume previous helm-gtags session
- Jump to a location based on context
- Find definitions
- Find references
- Present tags in current function only
- Create a tag database
- Jump to definitions in file
- Show stack of visited locations
- Manually update tag database
- Jump to next location in context stack
- Jump to previous location in context stack
- Jump to a file in tag database
- Enables =eldoc= in modes that otherwise might not support it.
- Enables =company complete= in modes that otherwise might not support it.

* Install

** GNU Global (gtags)
To use gtags, you first have to install [[https://www.gnu.org/software/global/download.html][GNU Global]].

You can install =global= from the software repository of your OS; however, many
OS distributions are out of date, and you will probably be missing support for
=pygments= and =exuberant ctags=, and thus support for many languages. We
recommend installing from source. If not for example to install on Ubuntu:

#+begin_src sh
  sudo apt-get install global
#+end_src

*** Install on OSX using Homebrew

#+begin_src sh options
  brew install global --with-pygments --with-ctags
#+end_src

*** Install on *nix from source
**** Install recommended dependencies
To take full advantage of global you should install 2 extra packages in
addition to global: pygments and ctags (exuberant). You can do this using
your normal OS package manager, e.g., on Ubuntu

#+BEGIN_SRC sh
  sudo apt-get install exuberant-ctags python-pygments
#+END_SRC

or e.g., Archlinux:

#+BEGIN_SRC sh
  sudo pacman -S ctags python-pygments
#+END_SRC

**** Install with recommended features
Download the latest tar.gz archive, then run these commands:

#+BEGIN_SRC sh
  tar xvf global-6.5.3.tar.gz
  cd global-6.5.3
  ./configure --with-exuberant-ctags=/usr/bin/ctags
  make
  sudo make install
#+END_SRC

**** Configure your environment to use pygments and ctags
To be able to use =pygments= and =ctags=, you need to copy the sample
=gtags.conf= either to =/etc/gtags.conf= or =$HOME/.globalrc=. For example:

#+begin_src sh
  cp gtags.conf ~/.globalrc
#+end_src

Additionally you should define GTAGSLABEL in your shell startup file e.g.
with sh/ksh:

#+begin_src sh
  echo export GTAGSLABEL=pygments >> .profile
#+end_src

** Emacs Configuration
To use this configuration layer, add it to your =~/.spacemacs=. You
will need to add =gtags= to the existing =dotspacemacs-configuration-layers=.

#+begin_src emacs-lisp
  (setq dotspacemacs-configuration-layers
        '( ;; ...
          gtags
           ;; ...
          ))
#+end_src

*** Disabling by default
If =ggtags-mode= is too intrusive you can disable it by default, by setting the
layer variable =gtags-enable-by-default= to =nil=.

#+BEGIN_SRC emacs-lisp
  (setq-default dotspacemacs-configuration-layers
    '((gtags :variables gtags-enable-by-default t)))
#+END_SRC

This variable can also be set as a file-local or directory-local variable for
additional control per project.

* Usage

Before using the =gtags=, remember to create a GTAGS database by the following
methods:

- From within Emacs, runs the command =helm-gtags-create-tags=, which is bound
  to ~SPC m g c~. If the language is not directly supported by GNU Global, you
  can choose =ctags= or =pygments= as a backend to generate tag database.

- From inside terminal, runs gtags at your project root in terminal:

#+BEGIN_SRC sh
  cd /path/to/project/root
  gtags
#+END_SRC

If the language is not directly supported by =gtags=, and you have not set the
GTAGSLABEL environment variable, use this command instead:

#+BEGIN_SRC sh
  gtags --gtagslabel=pygments
#+END_SRC

*** Language Support
**** Built-in languages
If you do not have =ctags= or =pygments= enabled gtags will only produce
tags for the following languages:

- asm
- c/c++
- java
- php
- yacc

**** Exuberant ctags languages
If you have enabled =exuberant ctags= and use that as the backend (i.e.,
GTAGSLABEL=ctags or --gtagslabel=ctags) the following additional languages
will have tags created for them:

- c#
- erlang
- javascript
- common-lisp
- emacs-lisp
- lua
- ocaml
- python
- ruby
- scheme
- vimscript
- windows-scripts (.bat .cmd files)

**** Universal ctags languages
If instead you installed you the newer/beta =universal ctags= and use that
as the backend (i.e., GTAGSLABEL=ctags or --gtagslabel=ctags) the following
additional languages will have tags created for them:

- clojure
- d
- go
- rust

**** Pygments languages (plus symbol and reference tags)
In order to look up symbol references for any language not in the built in
parser you must use the pygments backend. When this backend is used global
actually uses both ctags and pygments to find the definitions and uses of
functions and variables as well as "other symbols".

If you enabled pygments (the best choice) and use that as the backend (i.e.,
GTAGSLABEL=pygments or --gtagslabel=pygments) the following additional
languages will have tags created for them:

- elixir
- fsharp
- haskell
- octave
- racket
- scala
- shell-scripts
- tex

** Eldoc integration
This layer also integrates =ggtags= for its Eldoc feature. That means, when
writing code, you can look at the minibuffer (at the bottom) and see variable
and function definition of the symbol the cursor is on. However, this feature is
only activated for programming modes that are not one of these languages:

- C
- C++
- Common Lisp
- Emacs Lisp
- Python
- Ruby

Since these modes have better Eldoc integration already.

In addition, if output from =compile= (bound to ~SPC c C~), =shell-command=
(bound to ~SPC !~ and ~M-!~) or =async-shell-command= (bound to ~M-&~) commands
contains symbol in your project, you move cursor on such symbol and use any of
the gtags commands.

* Key bindings

| Key Binding | Description                                               |
|-------------+-----------------------------------------------------------|
| ~SPC m g c~ | create a tag database                                     |
| ~SPC m g f~ | jump to a file in tag database                            |
| ~SPC m g g~ | jump to a location based on context                       |
| ~SPC m g G~ | jump to a location based on context (open another window) |
| ~SPC m g d~ | find definitions                                          |
| ~SPC m g i~ | present tags in current function only                     |
| ~SPC m g l~ | jump to definitions in file                               |
| ~SPC m g n~ | jump to next location in context stack                    |
| ~SPC m g p~ | jump to previous location in context stack                |
| ~SPC m g r~ | find references                                           |
| ~SPC m g R~ | resume previous helm-gtags session                        |
| ~SPC m g s~ | select any tag in a project retrieved by gtags            |
| ~SPC m g S~ | show stack of visited locations                           |
| ~SPC m g u~ | manually update tag database                              |
