すまん。インデントがずれてしまった、、、


main = do
　　h <- openFile "test.log" ReadMode
　　hGetLinesTail h 10 0 >>= putStrLn
　　hClose h

blockSize = 128
hGetLinesTail h l pos = do
　　xs <- hGetContentsSeekFromEnd h pos
　　if l > (length$lines xs) then hGetLinesTail h l (pos + blockSize)
　　else return xs

hGetContentsSeekFromEnd h pos = do
　　hSeek h SeekFromEnd (-pos)
　　replicateM (fromInteger pos) (hGetChar h)