[Haskell-beginners] please critique my first stab at systems programming

[Daniel Fischer]

On Thursday 14 April 2011 12:14:59, Sean Perry wrote:
> The idea is to walk the disk looking for a signature, say NTFS or EXT.
> Since we do not know where the block containing this identifier is, we
> read the blocks in one at a time.

It may be more efficient to read the entire thing into a lazy ByteString 
instead of reading in very small strict chunks.
That also makes the eof check unnecessary.

> 
> Long term I would like to support command line arguments for the name of
> the file and the offset to start looking.
> 
> Comments on style, idioms, etc. welcomed. I am particularly interested
> in the hIsEOF check and if there is a better way to handle that.

$ cabal install stringsearch
-- faster searches for a pattern than provided by bytestring

import qualified Data.ByteString.Char8 as BC
import qualified Data.ByteString.Lazy as L
import qualified Data.ByteString.Lazy.Search as L

blockSize = 512

main = do
   stuff <- L.readFile "blocks"
   case L.indices (BC.pack "PART") stuff of
     [] -> putStrLn "Not Found."
     (i:_) -> putStrLn $ "Found at " ++ show (i `quot` blockSize) ++ "."
   putStrLn "Done."

> 
> import qualified Data.ByteString as B
> import qualified Data.ByteString.Char8 as BC
> import IO
> import System.IO
> 
> chunkSize = 512
> 
> searchForPattern handle pat = searchForPattern' 0 handle pat
> 
> searchForPattern' index handle pat = do
>     eof <- hIsEOF handle
>     if eof then return Nothing
>       else do
>         bytes <- B.hGet handle chunkSize
>         case BC.breakSubstring pat bytes of
>           (x, y) | BC.null y -> searchForPattern' (index + 1) handle pat
> 
>                  | otherwise -> return (Just index)
> 
> main = do
>     fromHandle <- openBinaryFile "blocks" ReadMode
>     result <- searchForPattern fromHandle (BC.pack "PART")
>     case result of
>       Nothing -> putStr "Not Found.\n"
>       Just n  -> putStr $ "Found at " ++ show n ++ ".\n"
>     hClose fromHandle
>     putStr "Done.\n"
>