Why isEOF doesn't work?

Question

Here is minimal complete example:

import Control.Monad
import System.IO

loop :: IO ()
loop =
    do line <- getLine
       putStrLn line
       eof  <- isEOF
       unless eof loop

main = loop

This program is supposed to read a line, print it out, stop if there is 'end of file' character in stdin. It doesn't leave the loop at all.

If I put eof <- isEOF before putStrLn line the program behaves very strange (try it!). I cannot get it at all: how putStrLn can possibly affect input stream and why doesn't the program terminate when I put 'end of file' character into stream (with Ctrl+D)?

---

Description of program's behavior when eof <- isEOF goes before putStrLn line:

After entering of a line, program does not print the entered line, but expects more input. As it gets more input, it starts to print previously entered lines. This is log of a test:

foo
boo
output: foo
bar
output: boo
baz
output: bar
< here I press Ctrl-D >
output: baz

Source:

import Control.Monad
import System.IO

loop :: IO ()
loop =
    do line <- getLine
       eof  <- isEOF
       putStrLn $ "output: " ++ line
       unless eof loop

main =
    do hSetBuffering stdin LineBuffering
       loop

Answer 1

From http://lambda.haskell.org/platform/doc/current/ghc-doc/libraries/haskell2010-1.1.1.0/System-IO.html#g:11:

NOTE: hIsEOF may block, because it has to attempt to read from the stream to determine whether there is any more data to be read.

The putStrLn doesn't affect the isEOF, but the isEOF prevents the program from getting to the putStrLn before more characters are available, or you have actually pressed ^D.

So you should never use hIsEOF/isEOF until the point in the program where you are ready to read more characters if there are any.