Ghc Tags Compiler Plugin

A Ghc Compiler Plugin which generates tags for each compiled module or component.

● Requirements

ghc >= 8.6

● Vim configuration

By default each generated tags file is put next to the corresponding *.cabal package file. If you just have a repo with a cabal file in the main directory vim default tags setting will work, if you have some modules in subdirectories you will either need to set:

:set tags+=*/tags

or pass an option to modify where tags are written, see below.

● Plugin options

The plugin accepts an only one option, which is a file path to the tags file. It can be an absolute path or relative (to the *.cabal package file rather than cabal.project file), for example:

-fplugin-opt=Plugin.GhcTags:../tags

This is useful if for cabal packages which are located in subdirectories.

● Configuration: Ghc / Cabal / Stack

Configuration of this plugin requires some familiarity with ghc packages. Check out documentation to use -plugin-package or -plugin-package-id. In the examples below we use -plugin-package=ghc-tags-plugin but specifying version -package=ghc-tags-plugin-0.0.0.0 (where 0.0.0.0 is the version you installed), might work better. You can use ghc-pkg latest ghc-tags-plugin (likely with appropriate --package-db flag) to check which version is available.

● Ghc

ghc -plugin-package=ghc-tags-plugin -fplugin=Plugin.GhcTags

You might also need to pass -package-db in which you installed the plugin.

● Cabal

Install the ghc-tags-plugin to cabal store with:

cabal install --lib ghc-tags-plugin

In cabal.project.local file add package stanza for every local package :

project some-project
    ghc-options: -package-db=PACKAGE_DB
                 -plugin-package=ghc-tags-plugin
                 -fplugin=Plugin.GhcTags

PACKAGE_DB is likely to be something like (for ghc-8.6.5) (all environment variables must be expanded):

${HOME}/.cabal/store/ghc-8.6.5/package.db

or on Windows (note the "" syntax)

"C:\\Users\\USER_NAME\\AppData\\Roaming\\cabal\\store\\ghc-8.6.5\\package.db

Note that you can also configure in this way non-local projects. You will likely want to pass -fplugin-opt=Plugin.GhcTags=PATH where PATH is an absolute path to your tags file.

● Stack

This is alternative method, which also could be modified for cabal (but it is not as nice as the previous method where you don't need to modify any files checked in vcs).

Add ghc-tags-plugin to build-depends in your *.cabal files. (You should hide it behind a cabal flag). And add these lines to stack.yaml file:

extra-deps:
  - git: https://github.com/coot/ghc-tags-plugin
    commit: a841dae7fb9c335101f7fa4187d02687d306f972

test-project: -plugin-package=ghc-tags-plugin
              -fplugin=Plugin.GhcTags

Check out an example here.

● Ghcid

If you follow the cabal configuration as above (using stack should work too)

ghcid --comaand "cabal repl project"

will update tags file as you modify your project.

● Makefile

The Makefile contains some useful commands, e.g. install, uninstall or reinstall the package in a package.db (by default into cabal store). This is mostly for development, but it could be useful in other scenarios as well.

● Exceptions

If a GHC plugin throws an exception, GHC stops. This plugin wraps IOExceptions, to make it obvious that it filed rather than GHC. This might mean you misconfigured the plugin (by passing wrong options). The result might look like this:

ghc: panic! (the 'impossible' happened)
  (GHC version 8.6.5 for x86_64-unknown-linux):
        GhcTagsPluginIOException ../: openFile: inappropriate type (Is a directory)

● Tips

• The plugin is safe for concurrent compilation. Setting jobs: $ncpus is safe. The plugin holds an exclusive (advisory) lock on the tags file. This will create synchronisation between threads / process which are using the same tags file.

• If you are working on a larger project, it might be better to not collect all tags in a single tags file, since at every compilation step one will need to parse a large tags file. Working with tag files of size 10000 tags (or ~1.5MB) is ok - though this will depend on the hardware.

● Security implications of compiler plugins

Such plugins can:

• run arbitrary IO;
• modify abstract syntax tree in some way; a malicious plugin could change some security parameter in your code exposing a security hole.

This plugin only reads & writes to tags file (and updates a shared mutable state) as of IO, and does not modify/ the syntax tree.