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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