Dear Haskellers, in several books (RWH, LYAH) I have found the statement that file I/O is more effective with bytestrings than with common strings. As I am doing mostly scientific programming and nearly every program of mine uses file I/O, I would like to check this. So I tried to write a small "benchmark": read a double matrix from file and write it to file again. Regarding to reading, the benefit of bytestrings was actually huge, it was about ten times faster than with strings. What refers to writing, however, I failed completely. Most important, I did not find a function which directly converts doubles to bytestrings. So the best I could figure out was the following ugly workaround using Text.Show.ByteString: import qualified Data.ByteString.Lazy as DL import qualified Text.Show.ByteString as BS import Data.Char import Data.List import Data.Array.Unboxed lineendw8 = DL.pack [fromIntegral (ord '\n')] blankw8 = DL.pack [fromIntegral (ord ' ')] showAll :: UArray (Int, Int) Double -> -- matrix Int -> -- number of rows Int -> -- number of columns DL.ByteString showAll mymatrix numrows numcols = foldr f lineendw8 [0..numrows-1] where f = showLine mymatrix numcols showLine :: UArray (Int,Int) Double -> -- matrix Int -> -- number of columns Int -> -- current row DL.ByteString -> -- accumulator DL.ByteString showLine mymatrix numcols row akku = let f col s = DL.append blankw8 $ DL.append (BS.runPut $ BS.showp (mymatrix!(row,col))) s in DL.append lineendw8 $ foldr f akku [0..numcols-1] main :: IO () main = do -- read file into UArray Int, Int) Double ... -- .... -- ... and write it to file again let bs = showAll mymatrix numrows numcols DL.writeFile "writeOut.dat" bs This was more in order to show goodwill than to present a solution, of course. It actually works, but, ugly as it is, it is by no means faster than the corresponding procedure with strings. So my question: what is the canonical way to write doubles to file? I guess this question must have been posed already hundred times before, so I would also appreciate very much a link to former answers ... Best regards Johannes Engels