maybe

问题 The lookup function in Data.Map and Data.IntMap currently return values wrapped in Maybe with the type signature lookup :: Ord k => k -> Map k a -> Maybe a It used to have the more general type of lookup :: (Monad m, Ord k) => k -> Map k a -> m a I realize the former likely reduces the need of extra type specification, but the latter would make it much more general and allow lookup to be used in list comprehensions. Is there any way to mimic this behavior with the newer version, or would I

问题 Both are terms whose type is the intersection of all types (uninhabited). Both can be passed around in code without failing until one attempts to evaluate them. The only difference I can see is that in Java, there is a loophole which allows null to be evaluated for exactly one operation, which is reference equality comparison ( == )--whereas in Haskell undefined can't be evaluated at all without throwing an exception. Is this the only difference? Edit What I'm really trying to get at with

问题 Both are terms whose type is the intersection of all types (uninhabited). Both can be passed around in code without failing until one attempts to evaluate them. The only difference I can see is that in Java, there is a loophole which allows null to be evaluated for exactly one operation, which is reference equality comparison ( == )--whereas in Haskell undefined can't be evaluated at all without throwing an exception. Is this the only difference? Edit What I'm really trying to get at with

问题 Is there an option (maybe) wokflow (monad) in the standrd F# library? I've found a dozen of hand-made implementations (1, 2) of this workflow, but I don't really want to introduce non-standard and not very trusted code into my project. And all imaginable queries to google and msdn gave me no clue where to find it. 回答1: There's no Maybe monad in the standard F# library. You may want to look at FSharpx, a F# extension written by highly-qualified members of F# community, which has quite a number

问题 Is there an option (maybe) wokflow (monad) in the standrd F# library? I've found a dozen of hand-made implementations (1, 2) of this workflow, but I don't really want to introduce non-standard and not very trusted code into my project. And all imaginable queries to google and msdn gave me no clue where to find it. 回答1: There's no Maybe monad in the standard F# library. You may want to look at FSharpx, a F# extension written by highly-qualified members of F# community, which has quite a number

问题 Why does Clojure, despite such an emphasis on functional paradigm, not use the Maybe / Option monad to represent optional values? The use of Option is quite pervasive in Scala, a functional programming language I use regularly. 回答1: Clojure is not statically typed, so doesn't need the strict this/that/whatever type declarations that are necessary in haskell (and, I gather, Scala). If you want to return a string, you return a string; if you return nil instead, that's okay too. "Functional"

问题 In Maybe applicative, <*> can be implemented based on fmap . Is it incidental, or can it be generalized to other applicative(s)? (<*>) :: Maybe (a -> b) -> Maybe a -> Maybe b Nothing <*> _ = Nothing (Just g) <*> mx = fmap g mx Thanks. See also In applicative, how can `<*>` be represented in terms of `fmap_i, i=0,1,2,...`? 回答1: It cannot be generalized. A Functor instance is unique: instance Functor [] where fmap = map but there can be multiple valid Applicative instances for the same type

问题 I wrote this function: appFunc :: Integer -> Integer -> Bool -> Maybe (Integer,Integer) appFunc i1 i2 b = if b then Just (i1,i2) else Nothing And then I use it as such in GHCi: > appFunc <$> Just 3 <*> Nothing <*> Just True Nothing Which is great because if at least one of the parameters is Nothing then the whole expression evaluates to Nothing . However, when all parameters are Just then I get a nested Maybe : > appFunc <$> Just 3 <*> Just 1 <*> Just False Just Nothing Ideally, I would like

问题 I had two functions f1:: String -> Int f2:: String -> Int f3:: String -> (Int,Int) f3 = f1 &&& f2 then they was changed to String -> Maybe Int f1:: String -> Maybe Int f2:: String -> Maybe Int f3:: String -> (Maybe Int,Maybe Int) Is there pointfree way to get function f4:: String -> Maybe (Int, Int) So if both f1 and f2 return Just, f4 will also return Just otherwise Nothing 回答1: import Control.Arrow import Control.Applicative h :: (Applicative f) => (a -> f b) -> (a -> f c) -> a -> f (b, c)

问题 I have a list like this: let foo = [Just 1, Just 2, Nothing, Just 3, Nothing, Nothing] By using catMaybes I can extract only the Just -constructed values: catMaybes foo -- [1,2,3] I'm now looking for a function that not only yields a list of Just s but also a count of Nothing s for a finite list by traversing it once. It should have a signature like this: catMaybesCount :: [Maybe a] -> ([a], Int) Note: This question was answered Q&A-style and therefore intentionally does not show any research