Why this haskell code cannot be compiled - haskell

Question

I was trying to compile a haskell game code, this code generates three thread, one for infinite loop, one for collecting user's interaction, one for triggering the events. However, the code cannot be compiled and I don't know why.
Here's the code:
module Main where
import Control.Concurrent
import Control.Monad
import System.IO
import System.Random
import Text.Printf
data Msg = C Char | Time
data Event = C Char | Time Char
main :: IO ()
main = do
hSetBuffering stdout NoBuffering
hSetBuffering stdin NoBuffering
hSetEcho stdin False
-- shared resources
chan <- newEmptyMVar
removedDigits <- newEmptyMVar
unmatchedDigits <- newEmptyMVar
numberOfGuesses <- newEmptyMVar
--starting the generating thread and the user thread
forkIO $ generatingThread chan
forkIO $ userThread chan
--the main loop
if mainloop chan == True then "Congratulations! You won!" else "Better luck next time!"
return()
mainloop :: Chan c -> Bool
let mainloop = do
if length unmatchedDigits >= 10
then return False
Event <- readChan c
if Event == "timer"
then unmatchedDigits ++ param
else if testGuessedNumber param unmatchedDigits == True
then
removeMatchedDigit
if length unmatchedDigits == 0
then return True
mainloop c
-- Generating Thread aka event thread generating the random numbers
generatingThread :: Chan msgChan -> IO ()
generatingThread msgChan = forever $ do
publishTimerEvent msgChan 1000000
publishTimerEvent :: Chan msgChan -> Int delay ()
publishTimerEvent msgChan = do
c <- getRandomChar
putMVar msgChan ("timer" c)
threadDelay newDelay
velocity <- 0.9
if delay * velocity < 100
then newDelay <- 100
else newDelay <- delay * velocity
publishTimerEvent msgChan newDelay
getRandomChar :: Char c ()
getRandomChar = do
i <- randomRIO (0,9)
let c = "0123456789" !! i
return c
-- User Thread
userThread :: MVar Msg -> IO ()
userThread chan = forever $ do
c <- getChar
putMVar chan (C c)
showStr(show c)
testGuessedNumber :: Int -> Int -> Bool
testGuessedNumber a b
| a == b = True
| otherwise = False
-- Shows the given string at the left edge of the current terminal line after
-- having blanked out the first 20 characters on that line.
showStr :: String -> IO ()
showStr s = putStr ("\r" ++ replicate 20 ' ' ++ "\r" ++ s)
The error is "test.hs:36:3: error: parse error on input ‘Event’"

Answer 1

Variable names can not begin with uppercase letters, such as Event. Try renaming the variable to something like event.

Answer 2

In Haskell, all if ... then ... else blocks must have all their components; what would the result be otherwise?
The problem is that the compiler was expecting an else, but it actually got Event. That said, you have more problems than a simple parse error. return does not do what you think it does. For example, this code will print hi.
main = do
return ()
putStrLn "hi"
The return function simply lifts a value into a monad, it doesn't stop the computation or anything like that. Here is what your probably want to have:
...
if length unmatchedDigits >= 10
then return False
else do
Event <- readChan c
if Event == "timer"
then ...
else ...
This way, nothing happens after the if block, so the function just ends there, with the last value as False (if length unmatchedDigits >= 10) or continues on properly (if length unmatchedDigits < 10).
You also almost certainly don't want to use Event (uppercase E), because that would mean it is a data constructor. You probably meant event (lowercase e), which is just an ordinary variable name.
Additionally: This is very, very non-idiomatic Haskell. You definitely don't need MVars in this situation, and certainly not four of them. Chans are not the same thing as MVars, and you don't need either one unless you are doing heavy-duty multi-threading. I highly recommend you completely rewrite this and try to minimize the amount of code that uses IO (it should be maybe 10-15 lines of IO code in this example, probably less).
This is not Java; you don't need to name your variables in your type signature (Chan msgChan -> Int delay ()), nor do you need to write wrapper functions for standard library functions to monomorphize their type. testGuessedNumber is literally the same function as (==).
You will have much more success if you revisit basic, pure function syntax and understand how problems are solved in Haskell than with trying to emulate another language. Read some LYAH or Real World Haskell.