Introduce a Spinner when no progress bar is displayed

Why?
====

With a spinner in place, there's visual indication that something's
happening.

src/Unused/CLI/SearchWithoutProgress.hs

@@ -3,7 +3,10 @@ module Unused.CLI.SearchWithoutProgress
     ) where
 
 import Unused.TermSearch (search)
+import Unused.CLI.Spinner (startSpinner, stopSpinner)
 
 searchWithoutProgressBar :: [String] -> IO [String]
-searchWithoutProgressBar terms =
-    concat <$> mapM search terms
+searchWithoutProgressBar terms = do
+    putStr " "
+    sp <- startSpinner
+    concat <$> mapM search terms <* stopSpinner sp

src/Unused/CLI/Spinner.hs

@@ -0,0 +1,45 @@
+module Unused.CLI.Spinner
+    ( startSpinner
+    , stopSpinner
+    ) where
+
+import Control.Monad (forever)
+import Control.Concurrent (ThreadId, killThread, threadDelay, forkIO)
+import Unused.CLI.Util
+
+data Spinner = Spinner
+    { sSnapshots :: [String]
+    , sLength :: Int
+    , sDelay :: Int
+    , sColors :: [Color]
+    , sThreadId :: Maybe ThreadId
+    }
+
+startSpinner :: IO Spinner
+startSpinner = do
+    let s = buildSpinner
+    tid <- forkIO $ runSpinner 0 s
+    return $ s { sThreadId = Just tid }
+
+stopSpinner :: Spinner -> IO ()
+stopSpinner Spinner{ sThreadId = Nothing } = return ()
+stopSpinner Spinner{ sThreadId = Just tid } = killThread tid
+
+buildSpinner :: Spinner
+buildSpinner =
+    Spinner snapshots (length snapshots) 100000 colors Nothing
+  where
+    snapshots = ["⣾", "⣽", "⣻", "⢿", "⡿", "⣟", "⣯", "⣷"]
+    colors = cycle [Black, Red, Yellow, Green, Blue, Cyan, Magenta]
+
+runSpinner :: Int -> Spinner -> IO ()
+runSpinner i s = forever $ do
+    setSGR [SetColor Foreground Dull currentColor]
+    putStr currentSnapshot
+    cursorBackward 1
+    threadDelay $ sDelay s
+    runSpinner (i + 1) s
+  where
+    currentSnapshot = sSnapshots s !! (i `mod` snapshotLength)
+    currentColor = sColors s !! (i `div` snapshotLength)
+    snapshotLength = sLength s

unused.cabal

@@ -27,6 +27,7 @@ library
                      , Unused.CLI.SearchWithProgress
                      , Unused.CLI.SearchWithoutProgress
                      , Unused.CLI.Util
+                     , Unused.CLI.Spinner
   build-depends:       base >= 4.7 && < 5
                      , process
                      , parsec