Ghc-mod doesn't use my sandbox. What for?

Do you have a file cabal.sandbox.config

? And are you using the file .cabal

for your project?

If you have both of them, you will be able to use ghc-mod check ...

and it will work.

Another advantage of using a .cabal file is that you can use cabal repl

ghci cabal run

to invoke and to invoke runhaskell

with the correct command line parameters.

Update

Here's a recipe you can try to see when ghc-mod can find your sandboxed sandbox. Perhaps this will help you determine what is different from your setup.

Run in a clean folder:

$ mkdir foo
$ cd foo
$ cabal sandbox init
$ cabal get split
$ cd split-0.2.2
$ cabal sandbox init --sandbox=../.cabal-sandbox

      

        
        
        
      

    

Edit line 55 split.cabal to add heredoc

as a dependency.

Modify src / Data / List / Split.hs to use the module Text.Heredoc

:

{-# LANGUAGE QuasiQuotes #-}
...
import Text.Heredoc
...
foo :: String
foo = [here|this is a test|]

      

        
        
        
      

    

Make sure to install heredoc

:

$ cabal install --only-dependencies

      

        
        
        
      

    

Finally, this should work:

$ ghc-mod check ./src/Data/List/Split.hs

      

        
        
        
      

    

And it will work if you cd into a subdirectory:

$ cd src
$ ghc-mod check ./Data/List/Split.hs

      

        
        
        
      

    

However, ghc-mod won't work if you leave split.cabal

:

(back at the top level directory)
$ mv split.cabal split.cabal-old
$ ghc-mod check ./src/Data/List/Split.hs

      

        
        
        
      

    

In this case, I created a sandbox in the parent directory of our working directory, but everything should work if the initial sandbox was created like this:

$ mkdir foo
$ cd foo
$ mkdir sandbox-dir
$ cd sandbox-dir
$ cabal sandbox init
$ cd ..
$ cabal get split
$ cd split-0.2.2
$ cabal sandbox init --sandbox=../sandbox-dir/.cabal-sandbox