Stuart Hungerford <stuart.hungerford@gmail.com> wrote:

This is kind-of related to my earlier question on looking up functions by name. Suppose I have a module with a number of functions with the same signature:

[...]

I'd like to choose and run one of these functions randomly at run time. [...]

Again the lookup approach seems most reasonable. The cleanest way is to define a simple name type for your functions: data FuncIx = FuncA | FuncB deriving (Ord) instance Random FuncIx where ... funcA :: A -> B funcB :: A -> B funcs :: Map FuncIx (A -> B) funcs = M.fromList (zip [FuncA, FuncB] [funcA, funcB]) If you want to go for maximum speed instead: import qualified Data.Vector as V type FuncIx = Int ... funcs :: V.Vector (A -> B) funcs = V.fromList [funcA, funcB] randFunc :: (RandomGen g) => g -> (A -> B, g) randFunc = first (funcs V.!) . randomR (0, 1) Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

I think the MonadRandom package has a elegant solution for your problem: fromList :: MonadRandom<http://hackage.haskell.org/packages/archive/MonadRandom/0.1.6/doc/html/Control-Monad-Random-Class.html#t:MonadRandom>m => [(a, Rational<http://hackage.haskell.org/packages/archive/base/4.4.1.0/doc/html/Prelude.html#t:Rational>)] -> m a Example: fromList [(add, 1), (scale, 1), (rareFunction, 0.1)] On Sun, May 20, 2012 at 8:42 AM, Stuart Hungerford < stuart.hungerford@gmail.com> wrote:

Hi,

This is kind-of related to my earlier question on looking up functions by name. Suppose I have a module with a number of functions with the same signature:

scale :: Int -> Int -> Int

scale s x = s * x

add :: Int -> Int -> Int

add a x = a + x

...

I'd like to choose and run one of these functions randomly at run time. I can see I could use some kind of case expression:

op :: Int -> Int -> Int

op p x = case random(1,2) of 1 -> scale p x 2 -> add p x

Or some kind of pattern guards:

op p x | random(1,2) == 1 = scale p x | otherwise = add p x

Although that method won't work as is for more than two choices. Are these methods the most idiomatic way of randomly choosing a function? How hard would it be to use the machinery of the QuickCheck library for this, given it must be doing something similar in test suites?

Thanks,

Stu

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