我尝试进行惰性I / O，但是整个文件都被消耗了

Question

I am a Haskell newbie. I want to read only N characters of a text file into memory. So I wrote this code:

main :: IO()
main = do
  inh <- openFile "input.txt" ReadMode
  transformedList <- Control.Monad.liftM (take 4) $ transformFileToList inh
  putStrLn "transformedList became available"
  putStrLn transformedList
  hClose inh

transformFileToList :: Handle -> IO [Char]
transformFileToList h = transformFileToListAcc h []

transformFileToListAcc :: Handle -> [Char] -> IO [Char]
transformFileToListAcc h acc = do
  readResult <- tryIOError (hGetChar h)
  case readResult of
    Left e -> if isEOFError e then return acc else ioError e
    Right c -> do let acc' = acc ++ [transformChar c]
                  putStrLn "got char"
                  unsafeInterleaveIO $ transformFileToListAcc h acc'

My input file several lines, with the first one being "hello world", and when I run this program, I get this output:

got char
transformedList became available
got char
["got char" a bunch of times]
hell

My expectation is that "got char" happens only 4 times. Instead, the entire file is read, one character at a time, and only THEN the first 4 characters are taken.

What am I doing wrong?

Answer 1

I acknowledge I don't understand how unsafeInterLeaveIO works but I suspect the problem here is somehow related to it. Maybe with this example you are trying to understand unsafeInterLeaveIO , but if I were you I'd try to avoid its direct use. Here is how I'd do it in your particular case.

main :: IO ()
main = do
    inh <- openFile "input.txt" ReadMode
    charList <- replicateM 4 $ hGetChar inh
    let transformedList = map transformChar charList
    putStrLn "transformedList became available"
    putStrLn transformedList
    hClose inh

This should just read the first 4 characters of the file.

If you are looking for a truly effectful streaming solution, I'd look into pipes or conduit instead of unsafeInterLeaveIO .