[Haskell] Adding a Show instance to RWH's RandomState example.

Question

Hi, I have just typed in the RandomState example from real world haskell. It looks like this:

import System.Random
import Control.Monad.State

type RandomState a = State StdGen a

getRandom :: Random a => RandomState a
getRandom =
  get >>= \gen ->
  let (val, gen') = random gen in
  put gen' >>
  return val

getTwoRandoms :: Random a => RandomState (a, a)
getTwoRandoms = liftM2 (,) getRandom getRandom

It works, but the result doesn't get displayed. I get the error message:

No instance for (Show (RandomState (Int, Int)))
  arising from a use of `print' at <interactive>:1:0-38
Possible fix:
  add an instance declaration for (Show (RandomState (Int, Int)))
In a stmt of a 'do' expression: print it

I am having some trouble adding an instance for Show RandomState. Can anyone show me how this is done?

Thanks.

Answer 1

Since RandomState is a synonym for State and there isn't an instance of show defined for State, you won't be able to show it.

You would also not be able to derive show because State is just a wrapper for a function and Haskell has no way to define a show for functions that would be useful:

Prelude> show (+)

<interactive>:1:0:
    No instance for (Show (a -> a -> a))
      arising from a use of `show' at <interactive>:1:0-7
    Possible fix: add an instance declaration for (Show (a -> a -> a))
    In the expression: show (+)
    In the definition of `it': it = show (+)

EDIT: Forgot to add the other piece: GHCi is giving you that error because it uses show behind the scenes on the expressions you enter... REPL and all that.

Answer 2

For the sake of being explicit, as jberryman and the comments on the question imply: Something of type RandomState (a, a) is a function, not a value. To do anything with it, you want to run it with an initial state.

I'm guessing you want something like this:

> fmap (runState getTwoRandoms) getStdGen
((809219598,1361755735),767966517 1872071452)

This is essentially what the runTwoRandoms function a bit further in RWH is doing.