hie-wrapper deals with missing System GHC

Closes haskell/vscode-haskell#110
Closes #803

Sample log lines

2018-12-28 14:36:23.081677593 [ThreadId 4] - run entered for hie-wrapper(hie-wrapper) Version 0.4.0.1, Git revision 4f0c017 (dirty) (2286 commits) x86_64 ghc-8.6.3
2018-12-28 14:36:23.08184515 [ThreadId 4] - Current directory:/tmp/ff
2018-12-28 14:36:23.0846877 [ThreadId 4] - Cradle directory:/tmp/ff
2018-12-28 14:36:23.084807006 [ThreadId 4] - Using plain GHC version
2018-12-28 14:36:23.084980989 [ThreadId 4] - Project GHC version:No System GHC found
2018-12-28 14:36:23.085006736 [ThreadId 4] - hie exe candidates :["hie-No System GHC found","hie","hie"]
2018-12-28 14:36:23.085201395 [ThreadId 4] - found hie exe at:/home/alanz/.local/bin/hie
2018-12-28 14:36:23.085228858 [ThreadId 4] - args:["--lsp","-d","-l","/tmp/hie.log"]
2018-12-28 14:36:23.085252547 [ThreadId 4] - launching ....

2018-12-28 14:36:23.087026804 [ThreadId 4] - Using plain GHC version
2018-12-28 14:36:23.08731647 [ThreadId 4] - Mismatching GHC versions: Project is No System GHC found, HIE is 8.6.3
2018-12-28 14:36:23.087376738 [ThreadId 4] - Run entered for HIE(hie) Version 0.4.0.1, Git revision 4f0c017 (dirty) (2286 commits) x86_64 ghc-8.6.3
2018-12-28 14:36:23.087474913 [ThreadId 4] - Current directory:/tmp/ff
2018-12-28 14:36:23.087594646 [ThreadId 4] -

Author: alanz

app/HieWrapper.hs

@@ -83,7 +83,10 @@ run opts = do
 
   let
     hieBin = "hie-" ++ ghcVersion
-    backupHieBin = "hie-" ++ init (dropWhileEnd (/='.') ghcVersion)
+    backupHieBin =
+      case dropWhileEnd (/='.') ghcVersion of
+        [] -> "hie"
+        xs -> "hie-" ++ init xs
     candidates' = [hieBin, backupHieBin, "hie"]
     candidates = map (++ exeExtension) candidates'
 

src/Haskell/Ide/Engine/Plugin/Base.hs

@@ -105,22 +105,27 @@ hieGhcDisplayVersion = compilerName ++ "-" ++ VERSION_ghc
 
 getProjectGhcVersion :: IO String
 getProjectGhcVersion = do
-  isStackProject <- doesFileExist "stack.yaml"
+  isStackProject   <- doesFileExist "stack.yaml"
   isStackInstalled <- isJust <$> findExecutable "stack"
   if isStackProject && isStackInstalled
     then do
       L.infoM "hie" "Using stack GHC version"
       catch (tryCommand "stack ghc -- --numeric-version") $ \e -> do
         L.errorM "hie" $ show (e :: SomeException)
         L.infoM "hie" "Couldn't find stack version, falling back to plain GHC"
-        tryCommand "ghc --numeric-version"
+        getGhcVersion
     else do
       L.infoM "hie" "Using plain GHC version"
-      tryCommand "ghc --numeric-version"
+      getGhcVersion
 
   where
+    getGhcVersion = do
+      isGhcInstalled   <- isJust <$> findExecutable "ghc"
+      if isGhcInstalled
+        then tryCommand "ghc --numeric-version"
+        else return "No System GHC found"
+
     tryCommand cmd =
-      -- Drop '\n' from the output like "7.10.3\n", "8.4.3\n"
       init <$> readCreateProcess (shell cmd) ""
 
 hieGhcVersion :: String