Thanks, Joel.

Putting the type IO [()] in the main declaration and this as the final line of the main function does do the trick:

sequence [putStrLn $ show s | s <- newList]

But this is the kind of thing that makes Haskell types difficult for beginners to work with...

Geoffrey

On Mon, Mar 9, 2015 at 4:15 PM, Joel Williamson <joel.s.williamson@gmail.com> wrote:

main must have type IO a. Hoogle tells me that to convert [IO a] -> IO [a], you should use the function sequence. Try applying that to your final line.

On Mon, 9 Mar 2015 16:07 Geoffrey Bays <charioteer7@gmail.com> wrote:

My main function looks like this:

main :: [IO()]
main = do
    let stud1 = Student {name = "Geoff", average = -99.0, grades = [66,77,88]}
    let stud2 = Student {name = "Doug", average = -99.0, grades = [77,88,99]}
    let stud3 = Student {name = "Ron", average = -99.0, grades = [55,66,77]}
    let studList = [stud1,stud2]
    let newList = calcAvg studList
    [putStrLn $ show s | s <- newList]
    --putStrLn $ show (newList !! 0)
    --putStrLn $ show (newList !! 1)

With this final line, putStrLn $ show (newList !! 0), the type IO () in the function declaration compiles fine.

But with [putStrLn $ show s | s <- newList] as the final line, [IO ()] in the function declaration will not compile, I get this error:

    Couldn't match expected type `IO t0' with actual type `[IO ()]'

What does the declared type need to be for a final line of:
[putStrLn $ show s | s <- newList]  ???

Thanks,

Geoffrey

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