A binding for the native wcwidth. It's important that you setLocale before using it, like this:

 #!/usr/bin/env runhaskell

 import Text.Printf
 
 import System.Locale.SetLocale
 import Data.Char.WCWidth

 main                     =  do
   setLocale LC_ALL (Just "")
   sequence_ [ display c | c <- chars ]
  where
   chars                  =  [minBound..'A']
   display c = printf "%04x  %2d  %s\n" (fromEnum c) (wcwidth c) (show c)

The program file WCWidthTableaux.hs contains a more extensive example of using wcwidth.

Note that this binding to the native implementation gets certain characters wrong in obvious ways as well as ways that are problematic for indentation based languages. The ASCII tab should be assigned a width of 8, not -1; and one is likely to find -1 assigned to numerous obscure characters (for example, symbols from the Book of Changes).