adds isolate install feature to compiled ghc command

app/ghcup/Main.hs

@@ -186,6 +186,7 @@ data GHCCompileOptions = GHCCompileOptions
   , ovewrwiteVer :: Maybe Version
   , buildFlavour :: Maybe String
   , hadrian      :: Bool
+  , isolateDir   :: Maybe FilePath
   }
 
 data UpgradeOpts = UpgradeInplace
@@ -1010,6 +1011,15 @@ ghcCompileOpts =
     <*> switch
           (long "hadrian" <> help "Use the hadrian build system instead of make (only git versions seem to be properly supported atm)"
           )
+    <*> optional
+          (option
+            (eitherReader isolateParser)
+            (  short 'i'
+            <> long "isolate"
+            <> metavar "DIR"
+            <> help "install in an isolated directory instead of the default one, no symlinks to this installation will be made"
+            )
+           )
 
 
 toolVersionParser :: Parser ToolVersion
@@ -1979,6 +1989,7 @@ Report bugs at <https://gitlab.haskell.org/haskell/ghcup-hs/issues>|]
                             addConfArgs
                             buildFlavour
                             hadrian
+                            isolateDir
                 GHCupInfo { _ghcupDownloads = dls } <- lift getGHCupInfo
                 let vi = getVersionInfo (_tvVersion targetVer) GHC dls
                 when setCompile $ void $ liftE $

lib/GHCup.hs

@@ -1750,6 +1750,7 @@ compileGHC :: ( MonadMask m
            -> [Text]                   -- ^ additional args to ./configure
            -> Maybe String             -- ^ build flavour
            -> Bool
+           -> Maybe FilePath           -- ^ isolate dir
            -> Excepts
                 '[ AlreadyInstalled
                  , BuildFailed
@@ -1768,7 +1769,7 @@ compileGHC :: ( MonadMask m
                  ]
                 m
                 GHCTargetVersion
-compileGHC targetGhc ov bstrap jobs mbuildConfig patchdir aargs buildFlavour hadrian
+compileGHC targetGhc ov bstrap jobs mbuildConfig patchdir aargs buildFlavour hadrian isolateDir
   = do
     PlatformRequest { .. } <- lift getPlatformReq
     GHCupInfo { _ghcupDownloads = dls } <- lift getGHCupInfo
@@ -1838,12 +1839,18 @@ compileGHC targetGhc ov bstrap jobs mbuildConfig patchdir aargs buildFlavour had
     alreadyInstalled <- lift $ ghcInstalled installVer
     alreadySet <- fmap (== Just tver) $ lift $ ghcSet (_tvTarget tver)
     when alreadyInstalled $ do
+      case isolateDir of
+        Just isoDir ->
+          lift $ $(logWarn) [i|GHC #{prettyShow tver} already installed. Isolate installing to #{isoDir} |]
+        Nothing ->
           lift $ $(logWarn) [i|GHC #{prettyShow tver} already installed. Will overwrite existing version.|]
       lift $ $(logWarn)
         "...waiting for 10 seconds before continuing, you can still abort..."
       liftIO $ threadDelay 10000000 -- give the user a sec to intervene
 
-    ghcdir         <- lift $ ghcupGHCDir installVer
+    ghcdir <- case isolateDir of
+      Just isoDir -> pure isoDir
+      Nothing -> lift $ ghcupGHCDir installVer
 
     bghc <- case bstrap of
       Right g    -> pure $ Right g
@@ -1860,10 +1867,15 @@ compileGHC targetGhc ov bstrap jobs mbuildConfig patchdir aargs buildFlavour had
         pure (b, bmk)
       )
 
+    case isolateDir of
+      Nothing ->
+        -- only remove old ghc in regular installs
         when alreadyInstalled $ do
           lift $ $(logInfo) [i|Deleting existing installation|]
           liftE $ rmGHCVer tver
           
+      _ -> pure ()
+
     forM_ mBindist $ \bindist -> do
       liftE $ installPackedGHC bindist
                                (Just $ RegexDir "ghc-.*")
@@ -1872,11 +1884,15 @@ compileGHC targetGhc ov bstrap jobs mbuildConfig patchdir aargs buildFlavour had
 
     liftIO $ B.writeFile (ghcdir </> ghcUpSrcBuiltFile) bmk
     
+    case isolateDir of
+      -- set and make symlinks for regular (non-isolated) installs
+      Nothing -> do
         reThrowAll GHCupSetError $ postGHCInstall tver
-
         -- restore
         when alreadySet $ liftE $ void $ setGHC tver SetGHCOnly
         
+      _ -> pure ()
+
     pure tver
 
  where