Re: An IO Question from a Newbie

On Sun, 14 Sep 2003, Glynn Clements wrote:

Brandon Michael Moore wrote:

...

Hal was pretty terse, so I'll explain why switching to putStrLn will help.

stdout is line buffered.

At least by default (see hSetBuffering). That means output will only be flushed to the screen once a newline is written. Your prompt wasn't printed because it didn't have a newline, so it was buffered until the second print provided one (read from the user, by way of s).

This is hardly specific to Haskell. Try this C program:

But there's one significant difference between C and Haskell, which is applicable in the case of Matt's program. In C, any line-buffered output streams are automatically flushed when a read from an unbuffered or line-buffered stream can't be satisfied from its buffer.

Interesting. I didn't know this. Maybe we should match this behaviour, or provide a write-string-and-flush function. It seems like this issue is causing an undue amound of trouble.

Also, it seemed fairly clear from Matt's original message that:

a) he didn't want to have to force a new-line (he noted that doing so eliminated the problem), and

I should note here that there is a gnu readline binding distributed with GHC. It's undocumented, but it seems to follow the C API pretty closely, and you can make a decent interface using only two of the functions.

b) he understood the concept of flushing, but presumably didn't know how to do it in Haskell.

While we're on the subject, I'll point out a couple of other differences between the buffering in ANSI C's stdio library and Haskell's:

1. In Haskell, you can change the buffering mode at any point; in C, you have to change it before any data is read from or written to the stream, otherwise the behaviour is undefined.

2. For an input stream which is associated with a tty, changing the buffering mode may also change the terminal settings (setting it to unbuffered disables canonical mode while setting it to line-buffered or fully-buffered enables it).

-- Glynn Clements _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe