monads

问题 This is the State Monad code I am trying to figure out data State a = State (Int -> (a, Int)) instance Monad State where return x = State (\c -> (x, c)) State m >>= f = State (\c -> case m c of { (a, acount) -> case f a of State b -> b acount}) getState = State (\c -> (c, c)) putState count = State (\_ -> ((), count)) instance Show State where -- doesn't work show (State a) = show a -- doesn't work I am trying to make State as instance of Show so that I can see the action of getState and

问题 For my diagramming tool, I'd like to keep the code of the core model isolated from the GUI. In the following example, the "state " is passed around with vDiag , which is a Tvar . This is a design decision in wx. Now, For my diagramming tool, I 'd like the core model to be "stored" in a fgl Graph, (with complex types in it), and wx will be given only a view on it; say in this example, a list of points for read access when painting, and some functions to write when clicking, dragging, etc.. . I

问题 Is there a library or tools for testing the laws of a custom monad? My current hacked attempt goes something like this: Define Arbitrary1 , similar to Eq1 , Show1 etc. Define a helper type that wraps Arbitrary1 as Arbitrary . Define a test (for example) for monadic laws. Is any of this already implemented somewhere? {-# LANGUAGE RankNTypes, ScopedTypeVariables #-} import Data.Functor.Classes import Data.Proxy import Test.QuickCheck import Test.QuickCheck.Function import Test.QuickCheck.Poly

问题 I am not sure how to formulate this question scientifically exact, so I am just going to show you an example. I am using state in a StateT transformer. Underlying is IO . Inside the StateT IO operation I need to use alloca . However, I can't lift alloca to StateT IO because it expects an argument of type (Ptr a -> IO a) while I require it to work with an argument of (Ptr a -> StateT IO MyState a) . (However, this is a generic question about monad transformers rather than specific to IO ,

问题 A map that I am operating on has monadic keys (of type IO Double ). I need to use findMax on this map. Can I use liftM for this? Map.findMax $ Map.fromList [(f x, "X"), (f y, "Y"), (f z, "Z")] Here f x has type IO Double . 回答1: It doesn't really make sense to have IO -typed values as keys in a map. A value of type IO t for some type t can be thought of as a "program" that produces a value of type t each time it is run: you can run it multiple times and every time it may produce a different

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 hindent changed my code to: do download i inputFile onException (callProcess (List.head args) (List.tail args)) (removeFileIfExists name) `finally` removeFileIfExists inputFile I can't determine if the finally applies to the rest of the do block, or just the state beginning onException . According to this, If you see something unexpected in a list, like where, insert a closing brace before instead of a semicolon. I'm unsure if that rule is applying here. Does the `finally` apply to the rest

问题 I have this very simple function import qualified Data.ByteString.Lazy as B getJson :: IO B.ByteString getJson = B.readFile jsonFile readJFile :: IO (Maybe Response) readJFile = parsing >>= (\d -> case d of Left err -> return Nothing Right ps -> return (Just ps)) where parsing = fmap eitherDecode getJson :: IO (Either String Response) where jsonFile is a path to a file on my harddrive (pardon the lack of do-notation, but I found this more clear to work with) my question is; is there a way for

问题 The following short Haskell program is intended to count a list of items from a file. The version using foldl' works fine, but the version using the ST Monad gives a stack space overflow message. Clearly there's a space leak of some sort here, but I haven't been able to solve it. The really interesting part is that the ST monad is supposed to be doing in-place updates and shouldn't be letting resources grow like this, although that may only pertain to main memory and not stack space. Could

问题 The author here provides the following example usage of a do-monad to combine test generators: (require '[clojure.test.check.generators :as gen]) (require '[clojure.algo.monads :as m]) (m/defmonad gen-m [m-bind gen/bind m-result gen/return]) (def vector-and-elem (m/domonad gen-m [n (gen/choose 1 10) v (gen/vector gen/int n) e (gen/element v)] [v, e])) (gen/sample vector-and-elem) ([[0 -1 1 0 -1 0 -1 1] 0] [[1 1 3 3 3 -1 0 -2 2] 3] [[8 4] 8]... There commentator here asserts that this is a