Question

Total Haskell noob here. I have a simple function, and a main. I don’t have any idea what this error means:

Couldn't match expected type `IO t0' with actual type `Bool'
In the expression: main
When checking the type of the function `main'

when compiling the code:

is_instructor :: String -> Bool
is_instructor "Jeremy Erickson" = True
is_instructor x = False

main :: Bool
main = is_instructor "foo"

Answer 1

main is the thing that gets called when you run your programme. It is expected that a programme does in some way interact with the outside world (read input, print output, such things), therefore it’s reasonable that a main should have the type IO something. For reasons of type safety and simplicity, that is a requirement in Haskell, like main in Java must have type public static void main(String[] arrgh).

You probably wanted you value to be printed, so

main :: IO ()
main = print $ is_instructor "foo"

would be what you want.