readIL' should be defined differently. Try:
readIL' ans = do a <- ... b <- ... if a==0 && b==0 then return ans else readIL' ((a,b):ans)
(note that I added "return".) I don't know what the ...s should be, but they definitely should not be what they are: readDec is some kind of number reader, but it doesn't deal with actually getting input. Other people here will know more. One way is to use getStr (or is it getStrLn??), split it at the space, and then use read to get the numbers. However, I suspect there is a better way. Good luck.
Ah! You were right - I was confused as to the purpose of readDec and friends. I changed it to the following, which works: readIncompatList :: IO [(Player, Player)] readIncompatList = readIL' [] where readIL' ans = do ln <- getLine let [a, b] = map read (words ln) if a == 0 && b == 0 then return ans else readIL' ((a,b):ans) Thanks! Jyrinx jyrinx_list@mindspring.com (It occurs to me ... I love Haskell: that "let [a, b] = map read (words ln)" would be so much longer in C ... :-) )