writing a function to build a list of ints from user input
Tue, 6 May 2003 05:02:56 -0500
On Tue, May 06, 2003 at 09:05:46PM +0000, sashan wrote:
> I'm trying to write a function that builds a list from user input. This
> is what I've come up with so far.
> --a program that makes a list of numbers from user input
> makeList :: IO [Int]
> makeList = do s <- getLine
> if read s /= 0 then return (read s:makeList) else
> return 
> However it fails with the following error:
> Couldn't match `[a]' against `IO [Int]'
> Expected type: [a]
> Inferred type: IO [Int]
> In the second argument of `(:)', namely `makeList'
> In the first argument of `return', namely `((read s) : makeList)'
> It's trying to match a list, [a], with type IO [Int]. This causes it to
> fail. How do I get around this?
This is happening because makeList returns IO [Int], whereas the (:)
operator or only defined over [a]. It doesn't work for the same
reason that you can't use getLine as a String. This should work:
makeList :: IO [Int]
makeList = do s <- getLine
case read s of
0 -> return 
x -> do xs <- makeList
I changed the if to a case so that (read s) doesn't get called twice.
Hope that helps.