import System.IO
import qualified Data.ByteString as BS
import qualified Data.ByteString.Char8 as C8
import Control.Exception (try, SomeException)

-- Reading files requires checking most calls for errors.
-- This helper will streamline our error checks below.
check :: Either SomeException a -> IO a
check result = case result of
    Left e -> error $ show e
    Right value -> return value

main :: IO ()
main = do
    -- Perhaps the most basic file reading task is
    -- slurping a file's entire contents into memory.
    content <- check $ try $ BS.readFile "/tmp/dat"
    C8.putStr content

    -- You'll often want more control over how and what
    -- parts of a file are read. For these tasks, start
    -- by opening a file to obtain a Handle.
    handle <- check $ try $ openFile "/tmp/dat" ReadMode

    -- Read some bytes from the beginning of the file.
    -- Allow up to 5 to be read but also note how many
    -- actually were read.
    b1 <- check $ try $ BS.hGet handle 5
    let n1 = BS.length b1
    putStrLn $ show n1 ++ " bytes: " ++ C8.unpack b1

    -- You can also seek to a known location in the file
    -- and read from there.
    check $ try $ hSeek handle AbsoluteSeek 6
    b2 <- check $ try $ BS.hGet handle 2
    let n2 = BS.length b2
    putStrLn $ show n2 ++ " bytes @ 6: " ++ C8.unpack b2

    -- Other methods of seeking are relative to the
    -- current cursor position,
    check $ try $ hSeek handle RelativeSeek 4

    -- and relative to the end of the file.
    check $ try $ hSeek handle SeekFromEnd (-10)

    -- The bytestring package provides some functions that may
    -- be helpful for file reading. For example, reads
    -- like the ones above can be more robustly
    -- implemented with hGetSome.
    check $ try $ hSeek handle AbsoluteSeek 6
    b3 <- check $ try $ BS.hGetSome handle 2
    let n3 = BS.length b3
    putStrLn $ show n3 ++ " bytes @ 6: " ++ C8.unpack b3

    -- There is no built-in rewind, but
    -- hSeek handle AbsoluteSeek 0 accomplishes this.
    check $ try $ hSeek handle AbsoluteSeek 0

    -- The bytestring package implements efficient
    -- reading methods that may be useful both for its efficiency
    -- with many small reads and because of the additional
    -- reading methods it provides.
    b4 <- check $ try $ BS.hGet handle 5
    putStrLn $ "5 bytes: " ++ C8.unpack b4

    -- Close the file when you're done (usually this would
    -- be scheduled immediately after opening with
    -- bracket).
    hClose handle