cooperate with stack and cabal to find the package-db

[gelisam]

currently, hint's package database is configured in the same way ghc is, via environment variables and -package-db arguments. This is super old-school, nowadays people use a .cabal file to specify which packages they want their programs to see. It would be great if that just worked out of the box without having to jump through hoops in order to set the right settings. At the very least, how to specify package databases should be documented somewhere.

[yaitskov]

@gelisam, ghc-pkg looks at GHC_PACKAGE_PATH and --package-db argument, but unsafeRunInterpreterWithArgs (ghc 8.10.2) ignores both.

being in nix-shell cabal builds my app and ghc-pkg list prints path to the package database, but
I don't know how specify package database location for the compiled program.
Basic linking is working for the elf file, because optparse help command is executed, but hint feature is not working.

[gelisam]

ghc-pkg looks at GHC_PACKAGE_PATH and --package-db argument, but unsafeRunInterpreterWithArgs (ghc 8.10.2) ignores both.

What makes you say that? If I install e.g. the colour package via stack and then run a program which uses hint with and without the flags which point it to stack's package database, I get different results:

import Data.Foldable (for_)
import Language.Haskell.Interpreter as Hint
import Language.Haskell.Interpreter.Unsafe as Hint

-- |
-- >>> main
-- with args:
-- result:
-- <no location info>: error:
--     Could not find module ‘Data.Colour’
--     Use -v (or `:set -v` in ghci) to see a list of the files searched for.
-- with args:
--   -package-db
--   /Users/gelisam/.stack/snapshots/.../8.10.4/pkgdb
-- result:
-- Data.Colour.SRGB.Linear.rgb 0.0 0.0 0.0
main :: IO ()
main = do
  let packageDbConfigs = [ []
                         , ["-package-db", "/Users/gelisam/.stack/snapshots/.../8.10.4/pkgdb"]
                         ]
  for_ packageDbConfigs $ \packageDbConfig -> do
    putStrLn "with args:"
    for_ packageDbConfig $ \arg -> do
      putStrLn $ "  " ++ arg
    putStrLn "result:"
    r <- Hint.unsafeRunInterpreterWithArgs packageDbConfig $ do
      Hint.setImports ["Prelude", "Data.Colour"]
      Hint.eval "mempty :: Colour Double"
    case r of
      Left (WontCompile [GhcError errMsg]) -> do
        putStrLn errMsg
      Right s -> do
        putStrLn s

[yaitskov]

In my case -package-db and GHC_PACKAGE_PATH don't help anyhow.

GHC_PACKAGE_PATH="/nix/store/h1bikz8x8v5g88b0vk78xdncckivan4j-ghc-8.10.4-with-packages/lib/ghc-8.10.4/package.conf.d:" my-elf-file
in the app:

unsafeRunInterpreterWithArgs [ "-package-db", "/nix/store/h1bikz8x8v5g88b0vk78xdncckivan4j-ghc-8.10.4-with-packages/lib/ghc-8.10.4/package.conf.d", "-package" , "myapp" ] 

GhcException "cannot satisfy -package myapp

But replacing package.cache in user conf ref solves problem:

rm ~/.cabal/store/ghc-8.10.4/package.db/package.cache
ln -s /nix/store/h1bikz8x8v5g88b0vk78xdncckivan4j-ghc-8.10.4-with-packages/lib/ghc-8.10.4/package.conf.d/package.cache   ~/.cabal/store/ghc-8.10.4/package.db/

Interpreter is working.
Let's break it with "-no-user-package-conf"

unsafeRunInterpreterWithArgs [  "-no-user-package-conf" ]

Still working! So I conclude that unsafeRunInterpreterWithArgs understands -package arguments.

[gelisam]

I can only repeat once again that I do not recommend having code from package X interpret code which imports a module from package X :)