[Haskell] line-based interactive program

hi, i'd like to know how to write simply a line-based interactive program, that is one with which you have a 'talk'. a simple filter-like line-based program ca be written as: -- a filter program process an entire input to yield some output type FilterProgram = [Line] -> [Line] but i don't knwo how to handle input/output such that the output can be printed before all the input exists (the user enter some lines, the program writes a response based on those lines, and so on) what's the simplest/most elegant approach ? thanks, minh thu