Alert user if upgraded ghcup is shadowed by old ghcup

Also alerts if the binary is not in PATH at all.

Fixes #111

lib/GHCup.hs

@@ -1325,6 +1325,13 @@ upgradeGHCup dls mtarget force pfreq = do
                                                            destFile
                                                            Overwrite
   lift $ chmod_755 destFile
+
+  liftIO (isInPath destFile) >>= \b -> when (not b) $
+    lift $ $(logWarn) [i|"#{toFilePath (dirname destFile)}" is not in PATH! You have to add it in order to use ghcup.|]
+  liftIO (isShadowed destFile) >>= \case
+    Nothing -> pure ()
+    Just pa -> lift $ $(logWarn) [i|ghcup is shadowed by "#{toFilePath pa}". The upgrade will not be in effect, unless you remove "#{toFilePath pa}" or make sure "#{toFilePath destDir}" comes before "#{toFilePath (dirname pa)}" in PATH.|]
+
   pure latestVer
 
 

lib/GHCup/Utils/File.hs

@@ -407,6 +407,32 @@ searchPath paths needle = go paths
       else pure False
 
 
+-- | Check wether a binary is shadowed by another one that comes before
+-- it in PATH. Returns the path to said binary, if any.
+isShadowed :: Path Abs -> IO (Maybe (Path Abs))
+isShadowed p = do
+  let dir = dirname p
+  fn <- basename p
+  spaths <- catMaybes . fmap parseAbs <$> (liftIO getSearchPath)
+  if dir `elem` spaths
+  then do
+    let shadowPaths = takeWhile (/= dir) spaths
+    searchPath shadowPaths fn
+  else pure Nothing
+
+
+-- | Check whether the binary is in PATH. This returns only `True`
+-- if the directory containing the binary is part of PATH.
+isInPath :: Path Abs -> IO Bool
+isInPath p = do
+  let dir = dirname p
+  fn <- basename p
+  spaths <- catMaybes . fmap parseAbs <$> (liftIO getSearchPath)
+  if dir `elem` spaths
+  then isJust <$> searchPath [dir] fn
+  else pure False
+
+
 findFiles :: Path Abs -> Regex -> IO [Path Rel]
 findFiles path regex = do
   dirStream <- openDirStream (toFilePath path)