haskell

问题 I'm testing a REST server. I hit it in the IO monad and simulate it in State Db where Db tracks the supposed state of the server. The following function is supposed to run both versions and compare the results... check :: (Eq a, MonadState d s) => s a -> IO a -> s (IO Bool) -- or: check :: (Eq a, MonadState d s, MonadIO i) => s a -> i a -> s (i Bool) check _ _ = (return.return) False -- for now but when I try it with these simplest possible functions ... simReset :: State Db () realReset ::

问题 I am learning Haskell and want to use "readHex", which according to Hoogle has type: readHex :: Num a => ReadS a How do you "extract" a result from such a function? What's the most common way, pattern match against the right constructor ie, [(a,"")] ?? LiftM and lifting in general seems to make some sense, but I'm lost when it comes to "unwinding" the monadic stack. 回答1: To answer the general question in general terms, the only way to extract values from a data constructor is pattern matching

问题 Can't find solution to a simple problem, the answer should be obvious. How to render url with a query parameters inside hamlet template? I.e. @{ItemsR} will generate http://localhost:3000/items and how do I generate something like http://localhost:3000/items?page=10&sort=name ? 回答1: Yesod is RESTful, you should use arguments in url format (eg. /items/page/10/sortby/name ) if you wish use QueryString format then, you loss the Yesod type safe url management. Below example show how use different

问题 I have a many-to-many relationship between two entities. For example, let's think about a blogpost which can have multiple authors. This could be a simple model for it: share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase| Person name String age Int Maybe deriving Show Blogpost title String deriving Show PersonBlogpost person PersonId blogpost BlogpostId |] My idea is to have an "Edit" page for a Blogpost entity, and in it have a multiSelectField for the authors, and have

问题 I have some haskell code Im trying to work my way thourgh but I dont have understand what is going in it. type Bag a = a -> Int emptyB :: Bag a emptyB = \e -> 0 countB :: Eq a => Bag a -> a -> Int countB b e = b e I understand that the Bag type is a function that takes in a generic object and returns a Int and countB is basically a wrapper for Bag that gets the number of generic objects in that Bag. But I dont really understand anything past that. How do I modify whats in the bag? Or the bag

问题 This question already has an answer here : :sprint for polymorphic values? (1 answer) Closed 11 months ago . I am trying to figure out, how lazy evaluation works and I've tried as following: Prelude> a = [1,2,3,5,6] Prelude> b = map (\x -> x * 8) a Prelude> :sprint b b = _ Prelude> b [8,16,24,40,48] Prelude> :sprint b b = _ The question is, why the last line does not show the evaluated list? I evaluated a line before. 回答1: Because b is basically a function from Num a dictionary to [a] for

问题 I'm trying to create a process, and communicate with it via a handle that I provide outside the createProcess function: stdOutH <- openFile (logDir </> "stdout.log") ReadWriteMode hSetBuffering stdOutH LineBuffering (_, _, _, ph) <- createProcess $ (proc "someproc" []) { std_out = UseHandle stdOutH , std_err = UseHandle stdErrH } line <- hGetLine stdOutH putStrLn $ "Got " ++ line The "someproc" process spits a line out to the standard output, and I want to read it from the process that

问题 I wrote the following function. f :: Integer -> Integer f x = if (odd x) then 0 else (floor . logBase 2) x But the following compile-time error occurs: F.hs:2:31: No instance for (RealFrac Integer) arising from a use of floor' Possible fix: add an instance declaration for (RealFrac Integer) In the first argument of (.)', namely `floor' In the expression: floor . logBase 2 In the expression: (floor . logBase 2) x F.hs:2:39: No instance for (Floating Integer) arising from a use of logBase'

问题 Loading a = 2+2.0 from a .hs file in GHCi and doing :t a shows a :: Double . On the other hand, doing let b = 2+2.0 and :t b in GHCi shows b :: Fractional a => a . How are you able to deduce this from these two documents? 4.3.4 Ambiguous Types, and Defaults for Overloaded Numeric Operations https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-750004.3 2.4.8 Type defaulting in GHCi: https://downloads.haskell.org/~ghc/7.8.3/docs/html/users_guide/interactive-evaluation.html I

问题 I have a class that provides a globally unique identifier for types: class Named a where nameOf :: a -> (String,String,String) -- (Package, Module, Identifier) default nameOf :: (Generic a, Named' (Rep a)) => a -> (String,String,String) nameOf = nameOf' . from which almost works: >>> data D = C >>> instance Named D >>> nameOf C ("","Main","D") but I can't get the datatype's package with GHC.Generics : class Named' f where nameOf' :: f a -> (String,String,String) instance (Datatype t) => Named