Szymon Z??bkiewicz wrote:
The compiler tells me thats there's an error on line 10: "The last statement in a 'do' construct must be an expression"
I think, you have reached the point where treating do-notation as magic won't help you. Remember,
do nr1 <- read (prompt "enter 1. number: ") nr2 <- read (prompt "enter 2. number: ")
is syntactic sugar for
read (prompt "enter 1. number: ") >>= \nr1 -> read (prompt "enter 2. number: ") >>= \nr2 ->
and it obvious that something is missing after the last arrow. That's the expression the compiler is complaining about. After the translation, it is also completely clear, that there is no "variable" which is ever "declared" and could be "assigned". On a side note, using "trap values" like the special 0 is an ugly style inherited from C. You might want to get used to explicit representations for missing values. Compare this:
read_new :: Maybe (Int, Int) -> IO (Int, Int) read_new (Just ab) = return ab read_new Nothing = do n1 <- read_prompt "enter 1. number: " n2 <- read_prompt "enter 2. number: " return (n1, n2) where read_prompt p = prompt p >>= readIO
Also note the 'read_prompt' function; I'm pretty sure you got the types of 'prompt' and 'read' messed up, too. So in anticipation of your next question: 'read'ing the 'prompt' action is not the same as 'read'ing the result of the 'prompt' action. Only the latter makes sense. Udo. -- "Enthusiasm is contagious, and so is boredom." -- Paul Graham