monad-transformers | 易学教程

Making a custom monad transformer an instance of MonadError

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问题 I want to make my monad transformer to be an instance of MonadError if the transformed monad is an instance. Basically I want my transformer to behave as the built-in transformers do, for example there is a MonadError instance for StateT : MonadError e m => MonadError e (StateT s m) I tried doing this: instance MonadError e m => MonadError e (MyMonadT m) But GHC started complaining about undecidable instances, apparently the MTL library just enables undecidable instances, but is there any way

Adjoint functors determine monad transformers, but where's lift?

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问题 I'm intrigued by the construction described here for determining a monad transformer from adjoint functors. Here's some code that summarizes the basic idea: {-# LANGUAGE MultiParamTypeClasses #-} import Control.Monad newtype Three g f m a = Three { getThree :: g (m (f a)) } class (Functor f, Functor g) => Adjoint f g where counit :: f (g a) -> a unit :: a -> g (f a) instance (Adjoint f g, Monad m) => Monad (Three g f m) where return = Three . fmap return . unit m >>= f = Three $ fmap (>>=

Design of interface abstraction

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问题 Currently, I try to write a small game program (Skat) as a hobby project. Skat is a trick-taking game were two players play against a single player. As there are different kinds of players (lokal player, network player, computer, etc.), I wanted to abstract the interface to the player. My basic idea is to use a typeclass Player , that defines all kind of things, a player have to do and to know (playing a card, get notified about who won the trick, etc). Then, the whole game is just done by a

Non type-variable argument in the constraint: MonadError Failure m

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问题 I have defined a custom error type: data Failure = NetworkError Message | UserIsTooStupid Message | InvalidOperation Message | UnexpectedError Message type Message = String I am trying to use MonadError with my error type: loadJSON :: (Aeson.FromJSON v, MonadIO m, MonadError Failure m) => URI -> m v loadJSON uri = do body <- liftIO $ getResponseBody =<< simpleHTTP (getRequest uri) case Aeson.decode body of Just json -> return json Nothing -> throwError $ SerialisationError "Couldn't

How to transform disjunction of Future to Future of disjunction

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问题 I have the result of a method: val res: Future[Int] Xor Future[String] = getResult(x) and would like to transform it and use it as Future[Int Xor String] I could not extrapolate my use case from the herding cats blog and am not sure whether a monad transformer would be the right tool here, perhaps rather some form of traverse ? Xor from cats stands in for any disjunction. Scalaz \/ or stdlib Either would be fine as well (though I would prefer a biased disjunction). Thank you 回答1: Just as

How do you save a tree data structure to binary file in Haskell

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问题 I'm trying to save a simple (but quite big) Tree structure into a binary file using Haskell. The structure looks something like this: -- For simplicity assume each Node has only 4 childs data Tree = Node [Tree] | Leaf [Int] And here is how I need the data look on disk: Each node starts with four 32-bit offsets to it's children, then follow the childs. I don't care much about the leafs, let's say it's just n consecutive 32-bit numbers. For practival purposes I would need some node labels or

What purpose does the complexity of `Except` serve in Haskell?

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I understand (I think) that there is a close relationship between Either and Except in Haskell, and that it is easy to convert from one to the other. But I'm a bit confused about best practices for handling errors in Haskell and under what circumstances and scenarios I would choose one over the other. For example, in the example provided in Control.Monad.Except , Either is used in the definition type LengthMonad = Either LengthError so that calculateLength "abc" is Right 3 If instead one were to define type LengthMonad = Except LengthError then calculateLength "abc" would be ExceptT (Identity

Scotty Using MongoDB

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I'm relatively new to Haskell, and this is my first time working with monad transformers. I'd really appreciate some help. runQuery :: Pipe -> Query -> ActionM (Either Failure [Document]) runQuery pipe query = access pipe master "nutrition" (find query >>= rest) main = do pipe <- runIOE $ connect $ host "127.0.0.1" scotty 3000 $ do post "/" $ do b <- body let user :: Either String User = eitherDecode b case user of Left err -> text . pack $ "Could not decode the user:" ++ err ++ ":\n" ++ (show b) Right u -> do let query::Query = (select ["i_name" =: ["$in" =: map (unpack . name) (foods u)]]

Applicative instance for MaybeT m assumes Monad m

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问题 I've been using the Haxl monad (described here: http://www.reddit.com/r/haskell/comments/1le4y5/the_haxl_project_at_facebook_slides_from_my_talk), which has the interesting feature that <*> for its Applicative instance isn't the same as ap from Control.Monad. This is a key feature that allows it to do concurrent computations without blocking. For example, if hf and ha are long computations, then let hf :: Haxl (a -> b) = ... ha :: Haxl a = ... in do f <- hf a <- ha return (f a) will do them

Implementing an MFunctor instance for RVarT

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Is it possible to implement an MFunctor instance for RVarT ? So far I've come up with the following: {-# LANGUAGE RankNTypes #-} import Data.RVar -- from rvar import Control.Monad.Trans.Class (lift) -- from transformers hoistRVarT :: Monad m => (forall t. n t -> m t) -> RVarT n a -> RVarT m a hoistRVarT f rv = sampleRVarTWith (lift . f) rv However this can not be used as a definition of hoist for MFunctor , due to the Monad constraint on m incurred by lift . The problem is, that I couldn't find another way to lift the the resulting monad into RVarT without lift . But I think conceptually it