[Haskell-cafe] Re: powerSet = filterM (const [True, False]) ... is this obfuscated haskell?

[porges at porg.es]

2009/7/17 Gleb Alexeyev <gleb.alexeev at gmail.com>:
> On Jul 17, 2009 1:40pm, Thomas Hartman wrote:
>> my question to all 3 (so far) respondants is, how does your
>>
>> explanation explain that the result is the power set?
>>

Because powerset(s) = 2^s?

I was going to make some nice code but I ended up with this monster :D

    {-# LANGUAGE ScopedTypeVariables #-}

    import Control.Monad

    -- a more generic "if"
    gif p t f
        | p == maxBound = t
        | otherwise     = f

    -- this is filterM, but with the generic if
    collect _ [] = return []
    collect p (x:xs) = do
        flg <- p x
        ys <- collect p xs
        return (gif flg (x:ys) ys) -- just changed if -> gif

    -- list exponentiation -- first parameter is fake, just to get an 'a'
    expSet :: forall a b. (Bounded a, Enum a, Eq a) => a -> [b] -> [[b]]
    expSet _a = collect (\_-> values :: [a])

    values :: (Bounded a, Enum a) => [a]
    values = enumFromTo minBound maxBound

    data Trool = Un | Deux | Trois deriving (Bounded, Enum, Eq, Show)
    trool = undefined :: Trool
    bool = undefined :: Bool

    powerset = expSet bool

I feel dirty :P
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