Thread: Haskell question--handling EOF in interactive program

hello, i'm new haskell, , try use process files (i.e., tasks similar might done in awk). specifically, i'd process text input file until reaches eof printing output @ appropriate times, , maintain state it.

here's stab @ program kind of thing:

php code:

--simple interactive program with state, takes
--user input string, appends it to current state
--string, and prints the state string
--state string grows as program is run

main :: io ()
main = helper("") --initial state is empty string

helper :: string -> io ()
helper initialstate = do
       linein <- getline
       putstrln(snd (transition (linein, initialstate)))
       helper(fst (transition (linein, initialstate)))

--pure function to implement logic--translates
--input and current state to next state and
--output
transition :: (string, string) -> (string, string)
transition inpair = (nextstate, nextstate) where nextstate = (snd inpair) ++ (fst inpair) 


the problem @ end of input file, prints error message:

code:

interact.hs: <stdin>: hgetline: end of file

i figure eliminate message checking eof, i'm not sure how this. know of simple way this?

in advance

iseof

php code:

--simple interactive program with state, takes
--user input string, appends it to current state
--string, and prints the state string
--state string grows as program is run

import system.io

main :: io ()
main = helper("") --initial state is empty string

helper :: string -> io ()
helper initialstate = do end <- iseof
                         if end
                             then putstr ""
                             else do linein <- getline
                                     putstrln(snd (transition (linein, initialstate)))
                                     helper(fst (transition (linein, initialstate)))

--pure function to implement logic--translates
--input and current state to next state and
--output
transition :: (string, string) -> (string, string)
transition inpair = (nextstate, nextstate) where nextstate = (snd inpair) ++ (fst inpair) 


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