[Haskell-cafe] Type problems

[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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