Rustom Mody wrote:

I have this from Peyton Jones awkward squad paper

getTwoChars :: IO (Char,Char) getTwoChars = do c1 <- getChar c2 <- getChar return (c1,c2)

Can someone explain what is happening here? *Main> getTwoChars ab ('a','b')

This seems perfectly fine. The getTwoChars action is one that will result in a 2-tuple of Chars, when run. The getChar action is a value of type IO Char, so it's an action that, when run, yields a Char. While defining an IO action like getTwoChars you can use other actions to build it. In the context of IO each action is executed in sequence (the two getChars) you have there, and the "<-" arrow is used to give their results a name, if you care about them. The "return x" action is just the action, which will yield x when run, so "return (c1, c2)" will yield the tuple in conformance to getTwoChar's type.

*Main> getTwoChars a ('\n','a')

This one seems really odd. Assuming that you typed "a" and then pressed the Return key, I don't know why it happens for you. For me it doesn't happen. I get the expected ('a', '\n'). Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

On Thursday 27 October 2011, 16:48:44, Rustom Mody wrote:

I have this from Peyton Jones awkward squad paper

getTwoChars :: IO (Char,Char) getTwoChars = do c1 <- getChar c2 <- getChar return (c1,c2)

Can someone explain what is happening here? *Main> getTwoChars ab ('a','b') *Main> getTwoChars a ('\n','a')

Hmm, here I get Prelude> let getTwoChars :: IO (Char, Char); getTwoChars = do { x <- getChar; y <- getChar; return (x,y); } Prelude> getTwoChars ab('a','b') Prelude> getTwoChars ac('a','c') Prelude> Since your result pair appears on a different line, it seems that your input stream is line buffered and you have to press <Return> to make the input available to getChar. Then the entered newline isn't removed by the first getTwoChars, so it becomes the first Char gotten by the second invocation of getTwoChars. What's your OS and ghc version?

On Thu, Oct 27, 2011 at 10:48, Rustom Mody wrote:

Can someone explain what is happening here? *Main> getTwoChars ab ('a','b') *Main> getTwoChars a ('\n','a')

The code is doing exactly what it says. getTwoChars reads two characters --- *not* a line. So the next character waiting to be read is the newline, which is returned by the next getTwoChars. This may depend to some extent on the platform, as Unix defaults to a line-oriented input interface (at the OS level) but Windows to character-oriented input. -- brandon s allbery allbery.b@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms