Get Terminal width Haskell

Question

How to get the width of the terminal in Haskell?

Things I tried

System.Posix.IOCtl (could not figure out how to get it to work) 

This only has to work unix.

Thanks

Answer 1

If you don't want a dependency on ncurses, here's a wrapper of the appropriate ioctl() request using the FFI, based on the accepted answer of Getting terminal width in C?

TermSize.hsc

{-# LANGUAGE ForeignFunctionInterface #-}

module TermSize (getTermSize) where

import Foreign
import Foreign.C.Error
import Foreign.C.Types

#include <sys/ioctl.h>
#include <unistd.h>

-- Trick for calculating alignment of a type, taken from
-- http://www.haskell.org/haskellwiki/FFICookBook#Working_with_structs
#let alignment t = "%lu", (unsigned long)offsetof(struct {char x__; t (y__); }, y__)

-- The ws_xpixel and ws_ypixel fields are unused, so I've omitted them here.
data WinSize = WinSize { wsRow, wsCol :: CUShort }

instance Storable WinSize where
  sizeOf _ = (#size struct winsize)
  alignment _ = (#alignment struct winsize) 
  peek ptr = do
    row <- (#peek struct winsize, ws_row) ptr
    col <- (#peek struct winsize, ws_col) ptr
    return $ WinSize row col
  poke ptr (WinSize row col) = do
    (#poke struct winsize, ws_row) ptr row
    (#poke struct winsize, ws_col) ptr col

foreign import ccall "sys/ioctl.h ioctl"
  ioctl :: CInt -> CInt -> Ptr WinSize -> IO CInt

-- | Return current number of (rows, columns) of the terminal.
getTermSize :: IO (Int, Int)
getTermSize = 
  with (WinSize 0 0) $ \ws -> do
    throwErrnoIfMinus1 "ioctl" $
      ioctl (#const STDOUT_FILENO) (#const TIOCGWINSZ) ws
    WinSize row col <- peek ws
    return (fromIntegral row, fromIntegral col)

This uses the hsc2hs preprocessor to figure out the correct constants and offsets based on the C headers rather than hardcoding them. I think it's packaged with either GHC or the Haskell Platform, so chances are you'll have it already.

If you're using Cabal, you can add TermSize.hs to your .cabal file and it'll automatically know how to generate it from TermSize.hsc. Otherwise, you can run hsc2hs TermSize.hsc manually to generate a .hs file which you can then compile with GHC.