Freeze/help user triggered features

When the user presses h or ?, we print a short usage/help text listing
the interactive key bindings.

When the user presses f, we freeze the output text. I personally use
this to copy/paste stuff from the terminal output, that's quite
convenient. There's no visual feedback showing the output text is
paused at the moment. This might be a UX footgun ><

lib/NOM/IO.hs

@@ -9,14 +9,14 @@ import Data.ByteString.Char8 qualified as ByteString
 import Data.Text qualified as Text
 import Data.Time (ZonedTime, getZonedTime)
 import NOM.Error (NOMError)
-import NOM.Print (Config (..))
+import NOM.Print (Config (..), printHelp)
 import NOM.Print.Table as Table (bold, displayWidth, displayWidthBS, markup, red, truncate)
 import NOM.State (PrintNameStyle (..), PrintState (..), initPrintState)
 import NOM.Update.Monad (UpdateMonad, getNow)
 import Relude
 import Streamly.Data.Fold qualified as Fold
 import Streamly.Data.Stream qualified as Stream
-import System.Console.ANSI (SGR (Reset), setSGRCode)
+import System.Console.ANSI (SGR (Reset), hClearScreen, setSGRCode)
 import System.Console.ANSI qualified as Terminal
 import System.Console.Terminal.Size qualified as Terminal.Size
 import System.IO qualified
@@ -252,6 +252,10 @@ processTextStream config parser updater maintenance printerMay finalize initialS
   printerMay & maybe keepProcessing \(printer, output_handle) -> do
     linesVar <- newTVarIO 0
     let keepProcessingStdin :: IO ()
+    let toggleHelp :: IO () = atomically $ do
+          print_state <- readTMVar print_state_var
+          writeTMVar print_state_var $ print_state{printHelp = not print_state.printHelp}
+          writeTMVar new_user_input ()
         keepProcessingStdin = forever $ do
           key <- getKey
           case key of
@@ -260,15 +264,25 @@ processTextStream config parser updater maintenance printerMay finalize initialS
                 print_state <- readTMVar print_state_var
                 let print_state_style = if print_state.printName == PrintName then PrintDerivationPath else PrintName
                 writeTMVar print_state_var $ print_state{printName = print_state_style}
-                writeTMVar input_received ()
-            "?" -> do
+                writeTMVar new_user_input ()
+            "?" -> toggleHelp
+            "h" -> toggleHelp
+            "f" -> do
               atomically $ do
-                print_state <- takeTMVar print_state_var
-                putTMVar print_state_var $ print_state{printHelp = True}
-                writeTMVar input_received ()
+                print_state <- readTMVar print_state_var
+                writeTMVar print_state_var $ print_state{freeze = not print_state.freeze}
+                writeTMVar new_user_input ()
             _ -> pure ()
         writeToScreen :: IO ()
-        writeToScreen = writeStateToScreen (not config.silent) linesVar state_var print_state_var output_builder_var refresh_display_var maintenance printer output_handle
+        writeToScreen = do
+          print_state <- atomically $ readTMVar print_state_var
+          case (print_state.freeze, print_state.printHelp) of
+            (True, _) -> pure () -- Freezing the output, do not print anything.
+            (_, True) -> do
+              hClearScreen output_handle
+              ByteString.hPut output_handle printHelp
+              System.IO.hFlush output_handle
+            _ -> writeStateToScreen (not config.silent) printedLinesVar state_var print_state_var output_builder_var refresh_display_var maintenance printer output_handle
         keepPrinting :: IO ()
         keepPrinting = forever do
           runConcurrently

lib/NOM/Print.hs

@@ -1,4 +1,4 @@
-module NOM.Print (stateToText, showCode, Config (..)) where
+module NOM.Print (stateToText, showCode, printHelp, Config (..)) where
 
 import Data.Foldable qualified as Unsafe
 import Data.IntMap.Strict qualified as IntMap
@@ -588,3 +588,13 @@ printDuration diff
 
 timeDiffSeconds :: Int -> Text
 timeDiffSeconds = printDuration . fromIntegral
+
+printHelp :: ByteString
+printHelp =
+  "\n==================\n"
+    <> "Nix Output Monitor\n"
+    <> "==================\n"
+    <> "Hotkeys\n\n"
+    <> "n: toggle derivation name/derivation path print\n"
+    <> "f: toggle screen freeze\n"
+    <> "? or h: toggle help screen\n"

lib/NOM/State.hs

@@ -202,6 +202,7 @@ data PrintNameStyle = PrintName | PrintDerivationPath deriving stock (Show, Eq,
 data PrintState = MkPrintState
   { printName :: PrintNameStyle
   , printHelp :: Bool
+  , freeze :: Bool
   }
   deriving stock (Show, Eq, Ord, Generic)
 
@@ -210,6 +211,7 @@ initPrintState =
   MkPrintState
     { printName = PrintName
     , printHelp = False
+    , freeze = False
     }
 
 data NOMV1State = MkNOMV1State