関数型プログラミング言語Haskell Part11

11 名前：デフォルトの名無しさん mailto:sage [2009/09/12(土) 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？ --------------------------------------------------------------------------- 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 `>>='

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