09/09/12 16:14:07
質問していい？
以下のモナドのなかで、union は Ord a であるところの Set a にしか使えません、と怒られるんだが、どこで制約課したらいいの？
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import qualified Data.Set as S
data Distribution a =
Dist { sample :: a, support :: (S.Set a), expectation :: ((a -> Double) -> Double) }
always :: a -> Distribution a
always x = Dist { sample = x, support = S.singleton x, expectation = \f -> f x }
newtype Probability a = Prob { runProbability :: Distribution a }
instance Monad Probability where
p >>= f =
let x = runProbability p in
Prob $ Dist { sample = sample $ runProbability $ f (sample x),
support = S.fold (\d -> S.union $ support (runProbability $ f d)) (S.empty) (support x),
expectation = \g -> (expectation x) (\x -> (expectation (runProbability $ f x)) g) }
return a = Prob $ always a
----------------------------------------------------------------------------
Could not deduce (Ord b) from the context ()
arising from a use of `S.union'
Possible fix: add (Ord b) to the context of the type signature for `>>='