[Haskell-beginners] Choosing a function randomly...

[Ertugrul Söylemez]

Stuart Hungerford <stuart.hungerford at 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/
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