When possible, use package-id arg to ghc/runghc

Current mechanism of using GHC_PACKAGE_PATH for runghc and ghc commands does
not seem to work well when we have multiple versions of the same package. GHC
does not always pick up the packages in the same order as GHC_PACKAGE_PATH.

This fix determines the package-ids using ghc-pkg and then passes
package-ids on command line of ghc or runghc invocation. This works only when
the user explicitly passes --package to runghc or ghc commands. When --package
is not specified we have no easy way to determine what all packages will be
used by the file being compiled.

This will make sure that scripts which explicitly list all packages will
always run reliably even in presence of packages which have multiple instances
of the same version or multiple versions installed.

fixes #1957 (Requires all packages to be listed explicitly)

src/Stack/GhcPkg.hs

@@ -13,6 +13,7 @@ module Stack.GhcPkg
   (getGlobalDB
   ,EnvOverride
   ,envHelper
+  ,findGhcPkgField
   ,createDatabase
   ,unregisterGhcPkgId
   ,getCabalPkgVer

src/main/Main.hs

@@ -1,6 +1,7 @@
 {-# LANGUAGE TupleSections #-}
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE DeriveDataTypeable #-}
+{-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TemplateHaskell #-}
@@ -61,6 +62,7 @@ import           Stack.Coverage
 import qualified Stack.Docker as Docker
 import           Stack.Dot
 import           Stack.Exec
+import           Stack.GhcPkg (findGhcPkgField)
 import qualified Stack.Nix as Nix
 import           Stack.Fetch
 import           Stack.FileWatch
@@ -750,19 +752,30 @@ execCmd ExecOpts {..} go@GlobalOpts{..} =
                menv <- liftIO $ configEnvOverride config eoEnvSettings
                (cmd, args) <- case (eoCmd, eoArgs) of
                    (ExecCmd cmd, args) -> return (cmd, args)
-                   (ExecGhc, args) -> execCompiler "" args
+                   (ExecGhc, args) -> getGhcCmd menv eoPackages [] args
                     -- NOTE: this won't currently work for GHCJS, because it doesn't have
                     -- a runghcjs binary. It probably will someday, though.
                    (ExecRunGhc, args) ->
-                        let opts = concatMap (\x -> ["-package", x]) eoPackages
-                        in execCompiler "" (opts ++ ("-e" : "Main.main" : args))
+                        getGhcCmd menv eoPackages ["-e", "Main.main"] args
                munlockFile lk -- Unlock before transferring control away.
                exec menv cmd args
   where
-    execCompiler cmdPrefix args = do
-        wc <- getWhichCompiler
-        let cmd = cmdPrefix ++ compilerExeName wc
-        return (cmd, args)
+      -- return the package-id of the first package in GHC_PACKAGE_PATH
+      getPkgId menv wc name = do
+          mId <- findGhcPkgField menv wc [] name "id"
+          case mId of
+              Just i -> return (head $ words (T.unpack i))
+              -- should never happen as we have already installed the packages
+              _      -> error ("Could not find package id of package " ++ name)
+
+      getPkgOpts menv wc pkgs = do
+          ids <- mapM (getPkgId menv wc) pkgs
+          return $ concatMap (\x -> ["-package-id", x]) ids
+
+      getGhcCmd menv pkgs prefix args = do
+          wc <- getWhichCompiler
+          pkgopts <- getPkgOpts menv wc pkgs
+          return (compilerExeName wc, prefix ++ pkgopts ++ args)
 
 -- | Evaluate some haskell code inline.
 evalCmd :: EvalOpts -> GlobalOpts -> IO ()