Implement ghcup gc command

Fixes #241

app/ghcup/Main.hs

@@ -117,6 +117,7 @@ data Command
   | Interactive
 #endif
   | Prefetch PrefetchCommand
+  | GC GCOptions
 
 data ToolVersion = ToolVersion GHCTargetVersion -- target is ignored for cabal
                  | ToolTag Tag
@@ -145,6 +146,15 @@ data InstallOptions = InstallOptions
   , forceInstall :: Bool
   }
 
+data GCOptions = GCOptions
+  { gcOldGHC :: Bool
+  , gcProfilingLibs :: Bool
+  , gcShareDir :: Bool
+  , gcHLSNoGHC :: Bool
+  , gcCache :: Bool
+  , gcTmp :: Bool
+  }
+
 data SetCommand = SetGHC SetOptions
                 | SetCabal SetOptions
                 | SetHLS SetOptions
@@ -438,6 +448,16 @@ com =
              (progDesc "Prefetch assets"
              <> footerDoc ( Just $ text prefetchFooter ))
            )
+      <> command
+           "gc"
+            (info
+             (   (GC
+                     <$> gcP
+                 ) <**> helper
+             )
+             (progDesc "Garbage collection"
+             <> footerDoc ( Just $ text gcFooter ))
+           )
       <> commandGroup "Main commands:"
       )
     <|> subparser
@@ -542,6 +562,10 @@ Examples:
   ghcup prefetch ghc 8.10.5
   ghcup --offline install ghc 8.10.5|]
 
+  gcFooter :: String
+  gcFooter = [s|Discussion:
+  Performs garbage collection. If no switches are specified, does nothing.|]
+
 configFooter :: String
 configFooter = [s|Examples:
 
@@ -1122,6 +1146,28 @@ prefetchP = subparser
       )
   )
 
+gcP :: Parser GCOptions
+gcP =
+  GCOptions
+  <$> 
+    switch
+      (short 'o' <> long "ghc-old" <> help "Remove GHC versions marked as 'old'")
+  <*> 
+    switch
+      (short 'p' <> long "profiling-libs" <> help "Remove profiling libs of GHC versions")
+  <*> 
+    switch
+      (short 's' <> long "share-dir" <> help "Remove GHC share directories (documentation)")
+  <*> 
+    switch
+      (short 'h' <> long "hls-no-ghc" <> help "Remove HLS versions that don't have a corresponding installed GHC version")
+  <*> 
+    switch
+      (short 'c' <> long "cache" <> help "GC the GHCup cache")
+  <*> 
+    switch
+      (short 't' <> long "tmpdirs" <> help "Remove tmpdir leftovers")
+
 
 ghcCompileOpts :: Parser GHCCompileOptions
 ghcCompileOpts =
@@ -1979,6 +2025,13 @@ Report bugs at <https://gitlab.haskell.org/haskell/ghcup-hs/issues>|]
                       , FileDoesNotExistError
                       ]
 
+          let runGC =
+                  runAppState
+                  . runResourceT
+                  . runE
+                    @'[ NotInstalled
+                      ]
+
 
           -----------------------
           -- Command functions --
@@ -2698,6 +2751,20 @@ Report bugs at <https://gitlab.haskell.org/haskell/ghcup-hs/issues>|]
                             VLeft e -> do
                               runLogger $ logError $ T.pack $ prettyShow e
                               pure $ ExitFailure 15
+            GC GCOptions{..} ->
+              runGC (do
+                  when gcOldGHC rmOldGHC
+                  lift $ when gcProfilingLibs rmProfilingLibs
+                  lift $ when gcShareDir rmShareDir
+                  lift $ when gcHLSNoGHC rmHLSNoGHC
+                  lift $ when gcCache rmCache
+                  lift $ when gcTmp rmTmp
+                   ) >>= \case
+                            VRight _ -> do
+                                  pure ExitSuccess
+                            VLeft e -> do
+                              runLogger $ logError $ T.pack $ prettyShow e
+                              pure $ ExitFailure 27
 
 
           case res of