Add a way to make ghc-mod's execution directory customizable

[adimit]

This allows us to modify from where ghcmod is executed. Always executing
it from the folder the current file resides in can cause problems with,
for example, Yesod's scaffolding, where some of the splices expect files
to live in certain places relative to the compiler's working directory.

To explain, consider the following: use yesod init to create a new default Yesod project, and cd to the new project's directory. Then:

% ghc-mod check Settings/StaticFiles.hs
# no problems reported
% cd Settings
% ghc-mod check StaticFiles.hs
# returns: StaticFiles.hs:1:1:Exception when trying to run compile-time code:  static: getDirectoryContents: does not exist (No such file or directory)  Code: staticFiles staticDir

Making it possible to set ghc-mod's working directory explicitly allows the user to use Yesod's default scaffolding with ghcmod-vim. There might be another fix, but I can't think of any.

[eagletmt]

I'm not familiar with Yesod, but doesn't it generate a cabal file?
ghcmod-vim has a mechanism to automatically detect the cabal file and set the suitable working directory (see ghcmod#build_command() https://github.com/eagletmt/ghcmod-vim/blob/master/autoload/ghcmod.vim#L351).

[eagletmt]

I'm sorry, I have misunderstood what is the problem.

But I don't think it is a good idea to set a global variable.
I suppose it should be a buffer-local setting.
Anyway, I will merge your commit and maybe I will add some fixes.
Thank you.

[adimit]

Sorry I didn't write the last four days, I was terribly busy!

Yes, a buffer-local setting might be a better solution, since you might have different projects open in the same vim session. The documentation would have to be updated to direct the user to set something like

au FileType haskell let b:ghcmod_use_basedir = getcwd()

I could send a patch if you'd like.

[jgreene]

I've been trying to use ghcmod-vim with yesod and I'm getting errors like the above. I've pulled the latest and tried:
au FileType haskell let b:ghcmod_use_basedir = getcwd()
in my .vimrc. Is there something else I'm missing?

[adimit]

Have you started Vim from the root of the scaffold site? I.e., instead of doing something like cd Handler; vim Home.hs, you just do vim Handler/Home.hs. If you're starting Vim from the file browser it'll probably not start in the project's root directory and you need to issue something like :cd .. (as a Vim command) in order to change the current working directory. You can find out Vim's working directory with :pwd.

[ghost]

Why not do what ctrlp does: if there exists a .git directory somewhere consider it as the root ?