Why hIsEOF wait for completely full buffer while hGetChar doesn't ?

[Nobuo Yamashita]

Hello,

I am curious about reason why in GHC hIsEOF blocks in BlockBuffering
mode while hGetChar doesn't.

I wrote following three programs for an experiment.

  -- echo0.hs
  module Main where
  import System.IO
  main = hSetBuffering stdin (BlockBuffering (Just 5)) >> echo
  echo = getChar >>= putChar >> hFlush stdout >> echo

  -- echo1.hs
  module Main where
  import System.IO
  main = hSetBuffering stdin (BlockBuffering (Just 5)) >> echo
  echo = do eof <- isEOF
            if eof then return () 
            else getChar >>= putChar >> hFlush stdout >> echo

  -- as.hs
  module Main where
  import System.IO
  import Control.Concurrent
  main = putChar 'a' >> hFlush stdour >> threadDelay 1000000 >> main

And run next two commandlines

% runghc as.hs | runghc echo0.hs
% runghc as.hs | runghc echo1.hs

I had expected that the both behaviors were same; a character 
was output a second. But the former output a character a second, and
the latter output 5 characters a time every 5 seconds.

Are there any explanations ?

Best regards.

--
Nobuo Yamashita