You have to fix the type of 1 and 6, e.g. by writing
x <- randomRIO (1, 6) :: IO Int
or
x <- randomRIO (1, 6 :: Int)
GHCi defaults integral numbers to Int, that's why it works there.
On 24 July 2010 21:36, michael rice
This works:
Prelude System.Random> do { randomRIO (1,6) >>= (\x -> putStrLn $ "Value = " ++ show x) } Value = 5
So does this:
Prelude System.Random> do { x <- randomRIO (1,6); putStrLn $ "Value = " ++ show x } Value = 2
But not this:
1 import Control.Monad 2 import System.Random 3 4 foo :: IO () 5 foo = do 6 x <- randomRIO (1,6) 7 putStrLn $ "Value = " ++ show x
foo.hs:6:18: Ambiguous type variable `t' in the constraints: `Num t' arising from the literal `1' at foo.hs:6:18 `Random t' arising from a use of `randomRIO' at foo.hs:6:7-21 Probable fix: add a type signature that fixes these type variable(s) Failed, modules loaded: none. Prelude System.Random>
Or this:
1 import Control.Monad 2 import System.Random 3 4 foo :: IO () 5 foo = randomRIO (1,6) >>= (\x -> putStrLn $ "Value = " ++ show x)
foo.hs:5:17: Ambiguous type variable `t' in the constraints: `Num t' arising from the literal `1' at foo.hs:5:17 `Random t' arising from a use of `randomRIO' at foo.hs:5:6-20 Probable fix: add a type signature that fixes these type variable(s) Failed, modules loaded: none.
How to fix?
Michael
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