[Haskell-beginners] Reading input

[C.M.Brown]

Barry,

On another note, you may want to avoid monads altogether for this. You can
run this straight from ghci (replacing mainProg with main)

mainProg :: [Int] -> Int
mainProg arg = median lst

and then call mainProg with your desired list of stings

> mainProg [1,2,3,4]

Chris.


On Wed, 20 Aug 2008, Yitzchak Gale wrote:

> Hi Barry,
>
> Barry Burd wrote:
> > median :: [Int] -> Int
> > median lst =
> >  case medians lst of
> >    -- All the values in the result are the same, we just pick the first one
> >    x:xs -> x
> >
> > main = do
> >    putStr "Enter a list: "
> >    lst <- getLine
> >    median lst
>
> > I'm sure this is because Haskell isn't automatically changing a String
> > to a List of numbers. But how can I do this?
>
> Use the "read" function. Watch out, though - there is
> no error checking there. So if the user enters a string
> that does not have the right syntax for a Haskell list
> of integers, your program will halt with an error message.
>
> You have another problem - you used the "median"
> function as a step in a do block - but each step in a
> do block must have type "IO a" for some type a.
>
> Here's how you could write your main function:
>
> main = do
>    putStr "Enter a list: "
>    lst <- getLine
>    print $ median $ read lst
>
> Regards,
> Yitz
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>