3 files changed, 50 insertions, 6 deletions
diff --git a/src/CabalHelper/Compiletime/Cabal.hs b/src/CabalHelper/Compiletime/Cabal.hs
index 69817c7..17f4d7f 100644
--- a/src/CabalHelper/Compiletime/Cabal.hs
+++ b/src/CabalHelper/Compiletime/Cabal.hs
@@ -20,11 +20,12 @@ Description : Cabal library source unpacking
 License     : GPL-3
 -}
 
-{-# LANGUAGE DeriveFunctor, ViewPatterns, CPP #-}
+{-# LANGUAGE DeriveFunctor, ViewPatterns, OverloadedStrings, CPP #-}
 
 module CabalHelper.Compiletime.Cabal where
 
 import Data.Char
+import Control.Exception
 import Data.List
 import Data.Maybe
 import Data.Time.Calendar
@@ -34,13 +35,15 @@ import Data.Version
 import System.Directory
 import System.Exit
 import System.FilePath
+import System.IO
 import Text.Printf
 
-
+import qualified Data.ByteString as BS
+import qualified Data.ByteString.Char8 as BS8
 
 import CabalHelper.Compiletime.Types
 import CabalHelper.Compiletime.Process
-import CabalHelper.Shared.Common (replace, parseVer, parseVerMay)
+import CabalHelper.Shared.Common (replace, parseVer, parseVerMay, parsePkgIdBS)
 
 type UnpackedCabalVersion = CabalVersion' (CommitId, CabalSourceDir)
 type ResolvedCabalVersion = CabalVersion' CommitId
@@ -259,3 +262,18 @@ findCabalFile pkgdir = do
 
 bultinCabalVersion :: Version
 bultinCabalVersion = parseVer VERSION_Cabal
+
+readSetupConfigHeader :: FilePath -> IO (Maybe UnitHeader)
+readSetupConfigHeader file = bracket (openFile file ReadMode) hClose $ \h -> do
+  parseSetupHeader <$> BS.hGetLine h
+
+parseSetupHeader :: BS.ByteString -> Maybe UnitHeader
+parseSetupHeader header = case BS8.words header of
+  ["Saved", "package", "config", "for", pkgId ,
+   "written", "by", setupId,
+   "using", compId]
+    -> UnitHeader
+       <$> parsePkgIdBS pkgId
+       <*> parsePkgIdBS setupId
+       <*> parsePkgIdBS compId
+  _ -> Nothing
diff --git a/src/CabalHelper/Compiletime/Program/GHC.hs b/src/CabalHelper/Compiletime/Program/GHC.hs
index 644c6ec..547911f 100644
--- a/src/CabalHelper/Compiletime/Program/GHC.hs
+++ b/src/CabalHelper/Compiletime/Program/GHC.hs
@@ -112,7 +112,7 @@ listCabalVersions mdb = do
     True -> MaybeT $ logIOError "listCabalVersions" $ Just <$> do
       let mdbopt = ("--package-conf="++) <$> mdb_path
           args = ["list", "--simple-output", "Cabal"] ++ maybeToList mdbopt
-      catMaybes . map (fmap snd . parsePkgId . fromString) . words
+      catMaybes . map (fmap snd . parsePkgId) . words
                <$> readProcess' (ghcPkgProgram ?cprogs) args ""
     _ -> mzero
 
diff --git a/src/CabalHelper/Compiletime/Types.hs b/src/CabalHelper/Compiletime/Types.hs
index 9911aec..95eea9f 100644
--- a/src/CabalHelper/Compiletime/Types.hs
+++ b/src/CabalHelper/Compiletime/Types.hs
@@ -27,6 +27,7 @@ module CabalHelper.Compiletime.Types where
 
 import Cabal.Plan
   ( PlanJson )
+import Data.ByteString (ByteString)
 import Data.IORef
 import Data.Version
 import Data.Typeable
@@ -229,6 +230,25 @@ uComponentName Unit { uImpl=UnitImplV2 { uiV2ComponentNames=[comp] } } =
 uComponentName _ =
     Nothing
 
+-- | The @setup-config@ header. Note that Cabal writes all the package names in
+-- the header using 'Data.ByteString.Char8' and hence all characters are
+-- truncated from Unicode codepoints to 8-bit Latin-1.
+--
+-- We can be fairly confident that 'uhSetupId' and 'uhCompilerId' won\'t have
+-- names that cause trouble here so it's ok to look at them but user packages
+-- are free to have any unicode name.
+data UnitHeader = UnitHeader
+    { uhBrokenPackageId  :: !(ByteString, Version)
+      -- ^ Name and version of the source package. Don't use this, it's broken
+      -- when the package name contains Unicode characters. See 'uiPackageId'
+      -- instead.
+    , uhSetupId    :: !(ByteString, Version)
+      -- ^ Name and version of the @Setup.hs@ implementation. We expect
+      -- @"Cabal"@ here, naturally.
+    , uhCompilerId :: !(ByteString, Version)
+      -- ^ Name and version of the compiler this Unit is configured to use.
+    } deriving (Eq, Ord, Read, Show)
+
 newtype UnitId = UnitId String
     deriving (Eq, Ord, Read, Show)
 
@@ -300,7 +320,9 @@ data ProjInfo pt = ProjInfo
   } deriving (Show)
 
 data ProjInfoImpl pt where
-  ProjInfoV1 :: ProjInfoImpl 'V1
+  ProjInfoV1 ::
+    { piV1SetupHeader :: !UnitHeader
+    } -> ProjInfoImpl 'V1
 
   ProjInfoV2 ::
     { piV2Plan        :: !PlanJson
@@ -313,7 +335,11 @@ data ProjInfoImpl pt where
     } -> ProjInfoImpl 'Stack
 
 instance Show (ProjInfoImpl pt) where
-    show ProjInfoV1 = "ProjInfoV1"
+    show ProjInfoV1 {..} = concat
+      [ "ProjInfoV1 {"
+      , "piV1SetupHeader = ", show piV1SetupHeader, ", "
+      , "}"
+      ]
     show ProjInfoV2 {..} = concat
       [ "ProjInfoV2 {"
       , "piV2Plan = ", show piV2Plan, ", "