detect terminal width and use it for pretty-printed output

src/System/Terminal.hsc

@@ -0,0 +1,44 @@
+{-# LANGUAGE CPP #-}
+#ifndef WINDOWS
+{-# LANGUAGE ForeignFunctionInterface #-}
+#endif
+
+module System.Terminal
+( getTerminalWidth
+) where
+
+#ifndef WINDOWS
+import           Foreign
+import           Foreign.C.Types
+
+#include <sys/ioctl.h>
+#include <unistd.h>
+
+newtype WindowWidth = WindowWidth CUShort
+    deriving (Eq, Ord, Show)
+
+instance Storable WindowWidth where
+  sizeOf _ = (#size struct winsize)
+  alignment _ = (#alignment struct winsize)
+  peek p = WindowWidth <$> (#peek struct winsize, ws_col) p
+  poke p (WindowWidth w) = do
+    (#poke struct winsize, ws_col) p w
+
+foreign import ccall "sys/ioctl.h ioctl"
+  ioctl :: CInt -> CInt -> Ptr WindowWidth -> IO CInt
+#endif
+
+-- | Get the width, in columns, of the terminal if we can.
+getTerminalWidth :: IO (Maybe Int)
+#ifndef WINDOWS
+getTerminalWidth =
+  alloca $ \p -> do
+    errno <- ioctl (#const STDOUT_FILENO) (#const TIOCGWINSZ) p
+    if errno < 0
+      then return Nothing
+      else do
+        WindowWidth w <- peek p
+        return . Just . fromIntegral $ w
+#else
+getTerminalWidth = pure Nothing
+#endif

stack.cabal

@@ -187,6 +187,7 @@ library
                      System.Process.PagerEditor
                      System.Process.Read
                      System.Process.Run
+                     System.Terminal
   other-modules:     Hackage.Security.Client.Repository.HttpLib.HttpClient
   build-depends:     Cabal >= 2.0 && < 2.1
                    , aeson (>= 1.0 && < 1.2)
@@ -267,6 +268,7 @@ library
                    , store-core >= 0.4 && < 0.5
                    , annotated-wl-pprint
                    , file-embed >= 0.0.10
+  build-tools:       hsc2hs >= 0.68
   if os(windows)
     cpp-options:     -DWINDOWS
     build-depends:   Win32