[Haskell-cafe] Type problems

[Tobias Brandt]

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 <nowgate at yahoo.com> 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>
>
> 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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>
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