GHC_PACKAGE_PATH not visible from running code

[mgsloan]

This is a side effect of #295

Repro:

stack runghc test.hs, where "test.hs" is

{-# LANGUAGE OverloadedStrings #-}
import qualified Data.ByteString.Char8 as S8
import           Data.Monoid           ((<>))
import           IdeSession

main :: IO ()
main = do
    -- Initialization and providing some code
    config <- sessionConfigFromEnv
    sess <- initSession defaultSessionInitParams config
        { configLocalWorkingDir = Nothing }
    let upd = updateSourceFile "Main.hs" "import System.Environment\nmain = print =<< lookupEnv \"GHC_PACKAGE_PATH\""
           <> updateCodeGeneration True
    updateSession sess upd print -- print is used for progress updates
    -- Run the code
    ra <- runStmt sess "Main" "main"
    let loop = do
            res <- runWait ra
            case res of
                Left bs -> S8.putStr bs >> loop
                Right rr -> putStrLn $ "Run result: " ++ show rr
    loop

Expected: something like `Just "STACK:PACKAGE:DB:PATHS"``

Actual: Nothing

When this is fixed, a test like the above should be added to make sure it stays fixed

[urbanslug]

Oh yeah this is what I was talking about when I told you I had an issue with sessionConfigFromEnv https://hackage.haskell.org/package/ide-backend-0.10.0/docs/IdeSession.html#v:sessionConfigFromEnv. My solution to this was getting config in the program's entry point (when it is being called from the terminal) and the config around as an argument wherever it is needed internally.

Like so: https://github.com/urbanslug/wai-devel/blob/master/src/Devel.hs#L16

[mgsloan]

Hmm, I don't follow. Why is this a problem for wai-devel? The only ramifications for wai-devel is that the web application wouldn't see GHC_PACKAGE_PATH. So, this could matter if your web application invoked stack or GHC.

[urbanslug]

The issue wasn't that applications were unable to find GHC_PACKAGE_PATH but ide-backend (and consequently wai-devel) failed to properly load sessionConfigFromEnv during recompiles. Wai-devel calls for recompiles from within a function when it notices file changes. Therefore, when wai-devel looked up sessionConfigFromEnv when a recompile was kicked off internally it failed to find GHC_PACKAGE_PATH because ide-backend could not.

[mgsloan]

Ahh, yes, that's a good point! Currently sessionConfigFromEnv, won't work properly after one session has run (it clears the GHC_PACKAGE_PATH environment). I hadn't considered that, thanks for bringing it up.

[urbanslug]

Haha told you I had your back :smile: