Install script with cabal: check there is one ghc in $PATH

install/src/Cabal.hs

@@ -113,7 +113,6 @@ cabalInstallIsOldFailMsg cabalVersion =
     ++ versionToString requiredCabalVersion
     ++ "`."
 
-
 requiredCabalVersion :: RequiredVersion
 requiredCabalVersion | isWindowsSystem = requiredCabalVersionForWindows
                      | otherwise = [2, 4, 1, 0]

install/src/Env.hs

@@ -62,6 +62,22 @@ findInstalledGhcs = do
     -- filter out stack provided GHCs (assuming that stack programs path is the default one in linux)
     $ filter (not . isInfixOf ".stack" . snd) (knownGhcs ++ availableGhcs)
 
+showInstalledGhcs :: MonadIO m => [(VersionNumber, GhcPath)] -> m ()
+showInstalledGhcs ghcPaths = do
+  let msg = "Found the following GHC paths: \n"
+              ++ unlines
+                  (map (\(version, path) -> "ghc-" ++ version ++ ": " ++ path)
+                    ghcPaths
+                  )
+  printInStars msg
+
+checkInstalledGhcs :: MonadIO m => [(VersionNumber, GhcPath)] -> m ()
+checkInstalledGhcs ghcPaths = when (null ghcPaths) $ do
+  let msg = "No ghc installations found in $PATH. \n"
+             ++ "The script requires at least one ghc in $PATH to be able to build hie.\n"
+  printInStars msg
+  error msg
+
 -- | Get the path to a GHC that has the version specified by `VersionNumber`
 -- If no such GHC can be found, Nothing is returned.
 -- First, it is checked whether there is a GHC with the name `ghc-$VersionNumber`.

install/src/HieInstall.hs

@@ -78,34 +78,27 @@ defaultMain = do
       (\version -> phony ("hie-" ++ version) $ do
         need ["submodules"]
         need ["check"]
-        if isRunFromStack then do
+        if isRunFromStack then
           stackInstallHieWithErrMsg (Just version)
         else
           cabalInstallHie version
       )
-
-    phony "latest" (need ["hie-" ++ latestVersion])
-    phony "hie"  (need ["data", "latest"])
+
+    unless (null versions) $ do
+      phony "latest" (need ["hie-" ++ latestVersion])
+      phony "hie"  (need ["data", "latest"])
 
     -- stack specific targets
     -- Default `stack.yaml` uses ghc-8.8.2 and we can't build hie in windows
     -- TODO: Enable for windows when it uses ghc-8.8.3
-    when (isRunFromStack && not isWindowsSystem) $ do
-
+    when (isRunFromStack && not isWindowsSystem) $
       phony "dev" $ stackInstallHieWithErrMsg Nothing
 
     -- cabal specific targets
     when isRunFromCabal $ do
-
-      phony "ghcs" $ do
-        let
-          msg =
-            "Found the following GHC paths: \n"
-              ++ unlines
-                  (map (\(version, path) -> "ghc-" ++ version ++ ": " ++ path)
-                        ghcPaths
-                  )
-        printInStars msg
+      -- It throws an error if there is no ghc in $PATH
+      checkInstalledGhcs ghcPaths
+      phony "ghcs" $ showInstalledGhcs ghcPaths
 
     -- macos specific targets
     phony "icu-macos-fix"