Hook into interrupt to trigger other behavior

Why?
====

Unused hides the cursor and potentially does other things to the window that
may leave it in an odd state. This introduces a hook to run any state
cleanup, including re-enabling the cursor, when a user sends a SIGINT to
the program.

app/Main.hs

@@ -5,7 +5,7 @@ import System.IO (hSetBuffering, BufferMode(NoBuffering), stdout)
 import Unused.Parser (parseLines)
 import Unused.Types (ParseResponse, RemovalLikelihood(..))
 import Unused.ResponseFilter (withOneOccurrence, withOneFile, withLikelihoods, ignoringPaths)
-import Unused.CLI (SearchRunner(..), executeSearch, printParseError, printSearchResults, resetScreen)
+import Unused.CLI (SearchRunner(..), executeSearch, printParseError, printSearchResults, resetScreen, withInterruptHandler)
 
 data Options = Options
     { oSearchRunner :: SearchRunner
@@ -15,8 +15,9 @@ data Options = Options
     }
 
 main :: IO ()
-main = run =<< execParser
-    (parseOptions `withInfo` "Analyze potentially unused code")
+main = withInterruptHandler $
+    run =<< execParser
+        (parseOptions `withInfo` "Analyze potentially unused code")
 
 run :: Options -> IO ()
 run options = do

src/Unused/CLI/Util.hs

@@ -1,11 +1,31 @@
 module Unused.CLI.Util
     ( resetScreen
+    , withInterruptHandler
     , module System.Console.ANSI
     ) where
 
 import System.Console.ANSI
+import Control.Exception (throwTo)
+import System.Posix.Signals (Handler(Catch), installHandler, keyboardSignal)
+import Control.Concurrent (ThreadId, myThreadId)
+import System.Exit (ExitCode(ExitFailure))
 
 resetScreen :: IO ()
 resetScreen = do
     clearScreen
     setCursorPosition 0 0
+
+withInterruptHandler :: IO () -> IO ()
+withInterruptHandler body = do
+    tid <- myThreadId
+    _ <- installHandler keyboardSignal (Catch (handleInterrupt tid)) Nothing
+    body
+
+handleInterrupt :: ThreadId -> IO ()
+handleInterrupt tid = do
+    resetScreen
+    showCursor
+    throwTo tid $ ExitFailure code
+  where
+    code = signalToInt $ 128 + keyboardSignal
+    signalToInt s = read $ show s :: Int

unused.cabal

@@ -40,6 +40,7 @@ library
                      , regex-tdfa
                      , terminal-progress-bar
                      , ansi-terminal
+                     , unix
   default-language:    Haskell2010
 
 executable unused