[Haskell-beginners] consing an element to a list inside a file

[Daniel Fischer]

Am Freitag 12 Februar 2010 20:15:26 schrieb kane96 at gmx.de:
> the exercise looks like that:
>
> readMyCal :: IO MyCalendar
> readMyCal = do putStr "day: "
>                d <- readInt
>                putStr "month: "
>                m <- readMonth
>                putStr "year: "
>                y <- readInt
>                let mydate = (d,m,y)
>                if not (legalDate mydate) then
>                    do putStrLn "wrong"
>                    --COMPLETE-1--
>                else
>                    --COMPLETE-2--
>
> COMPLETE-1: the user can give in a new date. I did it by simple calling
> the function again

Reasonable.

> COMPLETE-2: the user can choose if he wants to enter
> a new. If not, all dates the user has put in should be put out as a
> calendar (a list of dates)
>

Okay, that's not nice. But since you have no global variables to store the 
list of dates entered in (yes, there are ways to have global variables, but 
it's not a good idea unless you really have to), call

readMoreDates :: [MyDate] -> IO MyCaledar
readMoreDates previous = do ...

(code duplication where it isn't really necessary, but what the heck).