Hlint data-files can not be easily distributed

[fendor]

hlint demands that its data-file locations are known at compile time. If you remove these files, for example hlint.yaml, hlint will fail with a message and lets hie crash

hlint: user error (Failed to find requested hint files:
  ~/.cabal/store/ghc-8.4.4/hlint-2.1.15-8782d6ab3dc227b1c49b92fdeae8ecd837138c4d8f742c2f87f0c13927a32609/share/hlint.yaml
)

In this case, hlint has stored its data-files in ~/.cabal/store.

Unfortunately, if you copy the executable hie instead of installing from source, hie itself chokes on that error, since the data-files are not copied, thus, not available. We should, at the very least, catch that error and avoid crash.

Following options to resolve this issue:

• Fix this upstream, hlint should just create this file in some location or use defaults.
• Provide all data-files in a custom datadir directory.
• Turn hlint into an official run-time dependency and demand a working installation.

This is a blocker for #1068

[samuelpilz]

Moreover: hie it crashes when hlint does, which is undesirable (in my opinion). Can this be fixed?

[nponeccop]

Is this a duplicate of #400?

[samuelpilz]

@nponeccop It does tackle a similar issue.

I am currently in the process of evaluating runtime-dependencies of hie and found out that the 2 issues described are still relevant.
However, the runtime-dependency for hlint should be tackled in a seperate issue to the one for cabal-helper.

[fendor]

I propose the following, slightly weird, solution:
Use the package file-embed to embed the content of the data-files at compile time into hie.
On startup, check if these files exist at a pre determined location, e.g. $XDG_CONFIG_HOME/haskell-ide-engine/hlint. If yes, supply it to hlint as the --data-dir. Otherwise, write the content of the embedded files into that directory and continue.

Pro:

• Solves the run-time dependency of hlint on its data-files

Con:

• Might be error prone.
• Increases size of hie binary.

[samuelpilz]

I would like to tackle this at install-time. My plan:

• in install.hs: call cabal get hlint, then store the hlint/data into $HIE_DIR/hlint-data
• in hie: call hlint with --datadir=$HIE_DIR/hlint-data

My suggestion is that HIE_DIR=$XDG_DATA_HOME/hie, which is $HOME/.local/share by default. When I am able to reliably build binary packages, the directories $XDG_DATA_DIRS (default to /usr/share:/usr/local/share) will also searched for hie/hlint-data content.
See: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

@mpickering @fendor is this fine?