If foo really behaves like some RNG, it must have some monadic type, or take as an additional input the state of the RNG.
Here's an example, using the system RNG (which lives in the IO monad):
import System.Random foo :: IO Int foo = randomRIO (1,5) -- random number between 1 and 5 main :: IO () main = do x <- foo y <- foo print $ x /= y
In main, we simply call foo twice, binding its results to x,y, which have type Int. Then, we can compare them as usual.
Note that we can not use foo /= foo since foo :: IO Int, an action returning an Int, and we can't compare actions, only basic data like integers. This is why we first execute the actions, and then compare their results.
The main could be written in one line exploiting some more advanced combinators
main = print =<< ((/=) <$> foo <*> foo) -- or main = print =<< liftA2 (/=) foo foo
but I would not focus on these at the beginning.
A more general example of a side-effecful computation:
foo :: IORef String -> IO Int foo s = do x <- readIORef s -- reading a "variable" print x -- output w <- getLine -- input writeIORef s ("qwerty" ++ x) -- writing a variable length <$> readIORef s main :: IO () main = do s <- newIORef "dummy" print =<< foo s
Intis not the type of a function. It's just a number.