Still on the ICFPC 2007 topic I am curious about one thing. If I read the file applying hGetContents to a ByteString (Lazy or Strict) or a String, it seems to read much faster than the version where I construct a sequence. main = do (arg1:args)<-getArgs hIn <- openFile arg1 ReadMode c <-BL.hGetContents hIn --Really Fast let dna = c r<-return (process dna) print (show (r)) main = do (arg1:args)<-getArgs hIn <- openFile arg1 ReadMode c <-hGetContents hIn let dna = fromList c --Kind of slow r<-return (process dna) print (show (r)) I think the "fromList" overhead will be compensated by the O(log(n)) functions on a Seq, against the O(n) counterparts on the strings. What are your considerations about using Data.Sequence?