Re: [Haskell-cafe] Generating random enums

16 Oct 2009

      I didn't try to compile this:

import Control.Arrow (first)
import System.Random (Random(..))

instance Random Dir where
     randomR (lo, hi) gen = first fromEnum (randomR (toEnum lo)  
(toEnum hi) gen)
     random gen = randomR (minBound, maxBound)

But something along those lines should help you I think.

-Ross

On Oct 16, 2009, at 3:36 PM, michael rice wrote:
...
What is the minimum I need to do to get this function to generate a  
three direction tuple?
Michael
=====================
import System.Random
import Data.Ord
data Dir
    = North
    | South
    | East
    | West
    deriving (Show, Read, Eq, Enum, Ord, Bounded)
threeDirs :: StdGen -> (Dir,Dir,Dir)
threeDirs gen =
  let (firstDir, newGen) = random gen
      (secondDir, newGen') = random newGen
      (thirdDir, newGen'') = random newGen'
  in (firstDir, secondDir, thirdDir)
=====================
GHCi, version 6.10.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
Prelude> :l dir.hs
[1 of 1] Compiling Main             ( dir.hs, interpreted )
dir.hs:15:29:
    No instance for (Random Dir)
      arising from a use of `random' at dir.hs:15:29-42
    Possible fix: add an instance declaration for (Random Dir)
    In the expression: random newGen'
    In a pattern binding: (thirdDir, newGen'') = random newGen'
    In the expression:
        let
          (firstDir, newGen) = random gen
          (secondDir, newGen') = random newGen
          (thirdDir, newGen'') = random newGen'
        in (firstDir, secondDir, thirdDir)
Failed, modules loaded: none.
Prelude>
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Re: [Haskell-cafe] Generating random enums

Ross Mellgren