This module provides the serial port interface.

import qualified Data.ByteString.Char8 as B
import System.Hardware.Serialport
let port = "COM3"          -- Windows
let port = "/dev/ttyUSB0"  -- Linux
s <- openSerial port defaultSerialSettings { commSpeed = CS2400 }
send s $ B.pack "AT\r"
recv s 10 >>= print
closeSerial s

Or use the experimental interface with standard handles:

import System.IO
import System.Hardware.Serialport
let port = "COM3"           -- Windows
let port = "/dev/ttyUSB0"   -- Linux
h <- hOpenSerial port defaultSerialSettings
hPutStr h "AT\r"
hGetLine h >>= print
hClose h

You don't need the get or set functions, they are used by openSerial