[Haskell-cafe] Type problems

[Daniel Fischer]

On Saturday 24 July 2010 21:36:14, michael rice wrote:
> 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>

The x has type

(Num a, Random a) => a

so GHC doesn't know which type to choose.
GHCi uses extended defaulting rules and picks Integer.

Fix, tell GHC which type to choose:

    putStrLn $ "Value = " ++ show (x :: Integer)

or

    x <- randomRIO (1,6) :: IO Integer

>
> 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