The main function complains about returning a monad without IO
import Debug.Trace
main = do
trace "Main function parses, and returns - " "0"
return ()
This throws an error like,
app.hs:3:1:
Couldn't match expected type ‘IO t0’ with actual type ‘[()]’
In the expression: main
When checking the type of the IO action ‘main’
If I am not mistaken, the module should work without returning. But with or without a return function, it doesn't work.
trace
is not an IO action! Its type:
trace :: String -> a -> a
so that the compiler will tell you what you define main
to be in the Monad of the list! So he complains that you define it as [()]
when it should be IO ()
.
Try using traceIO
(or just putStrLn
). Keep in mind that trace
- this is a debug function: it prints things unsafely and breaks out of the IO monad, which should never be done by a real working program.
(In general, you can avoid confusion by writing the type signatures yourself: always write your function main
as
main :: IO () main = do ...
and then the error you get is less confusing since GHC won't call the weird type.)
Since your string trace
returns a list (string "0", which is of type [Char]
), you are actually calling the list monad, not the IO monad. The function return
for the list monad makes a singleton list from its argument: in this case [()]
.
Change the second argument to trace
be an IO action instead of a list.