higher-order-functions

问题 I\'m trying to construct a function of type: liftSumthing :: ((a -> m b) -> m b) -> (a -> t m b) -> t m b where t is a monad transformer. Specifically, I\'m interested in doing this: liftSumthingIO :: MonadIO m => ((a -> IO b) -> IO b) -> (a -> m b) -> m b I fiddled with some Haskell wizardry libs and but to no avail. How do I get it right, or maybe there is a ready solution somewhere which I did not find? 回答1: This can't be done generically over all MonadIO instances because of the IO type

问题 Reading through this classic paper, I\'m stuck on paramorphisms. Unfortunately the section is quite thin, and the Wikipedia page doesn\'t say anything. My Haskell translation is: para :: (a -> [a] -> b -> b) -> b -> [a] -> b para f base = h where h [] = base h (x:xs) = f x xs (h xs) But I don\'t grok that -- I don\'t have any intuition for the type signature or the desired result. What\'s a paramorphism, and what are some useful examples in action? Yes, I\'ve seen these questions, but they

问题 How would you flip 90 degrees (transpose) a multidimensional array in PHP? For example: // Start with this array $foo = array( \'a\' => array( 1 => \'a1\', 2 => \'a2\', 3 => \'a3\' ), \'b\' => array( 1 => \'b1\', 2 => \'b2\', 3 => \'b3\' ), \'c\' => array( 1 => \'c1\', 2 => \'c2\', 3 => \'c3\' ) ); $bar = flipDiagonally($foo); // Mystery function var_dump($bar[2]); // Desired output: array(3) { [\"a\"]=> string(2) \"a2\" [\"b\"]=> string(2) \"b2\" [\"c\"]=> string(2) \"c2\" } How would you