monads

问题 I'm sure I am missing something very obvious here. Here's what I'm trying to achieve at a conceptual level: action1 :: (MonadIO m) => m [a] action1 = pure [] action2 :: (MonadIO m) => m [a] action2 = pure [1, 2, 3] action3 :: (MonadIO m) => m [a] action3 = error "should not get evaluated" someCombinator [action1, action2, action3] == m [1, 2, 3] Does this hypothetical someCombinator exist? I have tried playing round with <|> and msum but couldn't get what I want. I guess, this could be

问题 In Haskell, you can use the bind operator ( >>= ) like this: repli :: [a] -> [a] repli xs = xs >>= \x -> [x,x] *Main> repli [1,2,3] [1,1,2,2,3,3] I've read that flatMap is Scala's bind operator: def repli [A](xs: List[A]): List[A] = xs.flatMap { x => List(x,x) } scala> repli (List(1,2,3)) res0: List[Int] = List(1, 1, 2, 2, 3, 3) As a pedagogic exercise, I'm trying to add support for >>= to Scala: class MyList[T](list: List[T]) { def >>= [U](f: T => List[U]): List[U] = list.flatMap(f) }

问题 Consider filterM (\x -> [True, False]) [1, 2, 3] I just cannot understand the magic that Haskell does with this filterM use case. The source code for this function is listed below: filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a] filterM _ [] = return [] filterM p (x:xs) = do flg <- p x ys <- filterM p xs return (if flg then x:ys else ys) With this use case, p should be the lambda function (\x -> [True, False]) , and the first x should be 1 . So what does flg <- p x return? What exactly

问题 Consider filterM (\x -> [True, False]) [1, 2, 3] I just cannot understand the magic that Haskell does with this filterM use case. The source code for this function is listed below: filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a] filterM _ [] = return [] filterM p (x:xs) = do flg <- p x ys <- filterM p xs return (if flg then x:ys else ys) With this use case, p should be the lambda function (\x -> [True, False]) , and the first x should be 1 . So what does flg <- p x return? What exactly

问题 Consider filterM (\x -> [True, False]) [1, 2, 3] I just cannot understand the magic that Haskell does with this filterM use case. The source code for this function is listed below: filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a] filterM _ [] = return [] filterM p (x:xs) = do flg <- p x ys <- filterM p xs return (if flg then x:ys else ys) With this use case, p should be the lambda function (\x -> [True, False]) , and the first x should be 1 . So what does flg <- p x return? What exactly

问题 Consider filterM (\x -> [True, False]) [1, 2, 3] I just cannot understand the magic that Haskell does with this filterM use case. The source code for this function is listed below: filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a] filterM _ [] = return [] filterM p (x:xs) = do flg <- p x ys <- filterM p xs return (if flg then x:ys else ys) With this use case, p should be the lambda function (\x -> [True, False]) , and the first x should be 1 . So what does flg <- p x return? What exactly

问题 I'm reading a revealing example of using a bind operator: Just 5 >>= (\ x -> if (x == 0) then fail "zero" else Just (x + 1) ) returns Just 6 . I'm confused by the behaviour of fail and its usefulness in the example. When looking at the code I thought fail "zero" may have a meaning: program never gets to that point laziness something else. Then I realised that after a type cohesion, an exception becomes Nothing (documented here). Still confusing for me that without type enforcement fail is

问题 Is there a standard / optimized implementation of the following function that I'm writing (probably unnecessarily): filterFirstM :: (Monad m, Foldable t) => (a -> Bool) -> t m a -> m a filterFirstM predicate actions = foldlM fn Nothing actions where fn memo action = case memo of Just _ -> pure memo Nothing -> do x <- action pure $ if predicate x then (Just x) else Nothing Sample usage: filterFirstM (== 1) [pure 0 :: IO Int, pure 1, error "not evaluated"] == (pure 1) 来源： https://stackoverflow

问题 Is there a standard / optimized implementation of the following function that I'm writing (probably unnecessarily): filterFirstM :: (Monad m, Foldable t) => (a -> Bool) -> t m a -> m a filterFirstM predicate actions = foldlM fn Nothing actions where fn memo action = case memo of Just _ -> pure memo Nothing -> do x <- action pure $ if predicate x then (Just x) else Nothing Sample usage: filterFirstM (== 1) [pure 0 :: IO Int, pure 1, error "not evaluated"] == (pure 1) 来源： https://stackoverflow

问题 I am working on some bigger computation that requires use of mutable data in some critical moments. I want to avoid IO as much as I can. My model used to constist of ExceptT over ReaderT over State datatype, and now I want to replace State with mentioned ST . To simplify, let's assume I would like to keep single STRef with an Int during the whole computation, and let's skip the ExceptT outer layer. My initial idea was to put STRef s Int into ReaderT 's environment: {-#LANGUAGE Rank2Types#-} {