How to format numbers according to locale in Haskell?

Question

In Python I can use locale.format to pretty-print numbers according to locale setting:

>>> import locale
>>> locale.setlocale(locale.LC_ALL, "en_US.UTF-8")
'en_US.UTF-8'
>>> locale.format("%.2f",1234567.89,grouping=True)
'1,234,567.89'

How can I do the same in Haskell? I see that there are localeconv and setlocale bindings, but is there a generic pretty printer which respects Lconv?

Answer 1

I would say that if the library in question is missing then you could either write yourself one (obvious option, not easy) or write a binding for the needed function. For example, restricted binding for sprintf which allows to sprintf only doubles:

Double.hs:

{-# INCLUDE "double.h" #-}
{-# LANGUAGE ForeignFunctionInterface #-}
module Double (cPrintf) where

import Foreign
import Foreign.C.Types
import System.IO.Unsafe
import qualified Data.ByteString as B

foreign import ccall "double.h toString"
 c_toString :: CDouble -> (Ptr Word8) -> CInt -> IO CInt

buf = unsafePerformIO $ mallocBytes 64

cPrintf :: Double -> B.ByteString
cPrintf n = B.pack $ unsafePerformIO $ do
   len <- c_toString (realToFrac n) buf 64
   peekArray (fromIntegral len) buf

double.h:

int toString(double a, char *buffer, int bufferLen);

double.c:

#include <stdio.h>
#include "double.h"

int toString(double a, char *buffer, int bufferLen) {
 return snprintf(buffer, bufferLen, "%f", a);
}

Build as:

gcc -c double.c
ghc --make Main.hs double.o