imp: cli: use $PAGER when help output is taller than terminal

This works with hledger, hledger -h and hledger CMD -h.

hledger-lib/Hledger/Utils/IO.hs

@@ -19,6 +19,9 @@ module Hledger.Utils.IO (
   pprint,
   pprint',
 
+  -- * Viewing with pager
+  pager,
+
   -- * Command line arguments
   progArgs,
   outputFileOption,
@@ -57,7 +60,7 @@ import           Control.Monad (when)
 import           Data.FileEmbed (makeRelativeToProject, embedStringFile)
 import           Data.List hiding (uncons)
 import           Data.Maybe (isJust)
-import           Data.Text (Text)
+import           Data.Text (Text, pack)
 import qualified Data.Text.IO as T
 import qualified Data.Text.Lazy as TL
 import qualified Data.Text.Lazy.Builder as TB
@@ -74,6 +77,7 @@ import           System.IO
   (Handle, IOMode (..), hGetEncoding, hSetEncoding, hSetNewlineMode,
    openFile, stdin, stdout, stderr, universalNewlineMode, utf8_bom)
 import           System.IO.Unsafe (unsafePerformIO)
+import System.Pager
 import           Text.Pretty.Simple
   (CheckColorTty(CheckColorTty), OutputOptions(..), 
   defaultOutputOptionsDarkBg, defaultOutputOptionsNoColor, pShowOpt, pPrintOpt)
@@ -114,6 +118,11 @@ pprint' = pPrintOpt CheckColorTty prettyopts'
 
 -- "Avoid using pshow, pprint, dbg* in the code below to prevent infinite loops." (?)
 
+-- | Display the given text on the terminal, using the user's $PAGER if the text is taller 
+-- than the current terminal and stdout is interactive.
+pager :: String -> IO ()
+pager s = printOrPage $ pack s
+
 -- Command line arguments
 
 -- | The command line arguments that were used at program startup.

hledger-lib/package.yaml

@@ -58,6 +58,7 @@ dependencies:
 - microlens >=0.4
 - microlens-th >=0.4
 - mtl >=2.2.1
+- pager >=0.1.1.0
 - parser-combinators >=0.4.0
 - pretty-simple >4 && <5
 - regex-tdfa

hledger/Hledger/Cli/Commands.hs

@@ -289,7 +289,7 @@ knownCommands = sort . commandsFromCommandsList . drop 1 $ commandsList progname
 -- extra addons will be added under Misc.
 printCommandsList :: String -> [String] -> IO ()
 printCommandsList progversion addonsFound =
-    putStr . unlines . concatMap adjustline $
+    pager . unlines . concatMap adjustline $
     commandsList progversion (map ('+':) unknownCommandsFound)
   where
     commandsFound = map (' ':) builtinCommandNames ++ map ('+':) addonsFound

hledger/Hledger/Cli/Main.hs

@@ -140,13 +140,13 @@ main = do
     isExternalCommand    = not (null cmd) && cmd `elem` addons -- probably
     isBadCommand         = not (null rawcmd) && null cmd
     hasVersion           = ("--version" `elem`)
-    printUsage           = putStr $ showModeUsage $ mainmode addons
+    printUsage           = pager $ showModeUsage $ mainmode addons
     badCommandError      = error' ("command "++rawcmd++" is not recognized, run with no command to see a list") >> exitFailure  -- PARTIAL:
     hasHelpFlag args1     = any (`elem` args1) ["-h","--help"]
     hasManFlag args1      = (`elem` args1) "--man"
     hasInfoFlag args1     = (`elem` args1) "--info"
     f `orShowHelp` mode1
-      | hasHelpFlag args = putStr $ showModeUsage mode1
+      | hasHelpFlag args = pager $ showModeUsage mode1
       | hasInfoFlag args = runInfoForTopic "hledger" (headMay $ modeNames mode1)
       | hasManFlag args  = runManForTopic "hledger" (headMay $ modeNames mode1)
       | otherwise        = f