haskell

问题 Playing around with the ReadArgs package, it seems that it does not support single-argument situations. {-# LANGUAGE ScopedTypeVariables #-} import ReadArgs (readArgs) main = do (foo :: Int) <- readArgs print foo The error is (when using version 1.0): No instance for (ReadArgs.ArgumentTuple Int) arising from a use of `readArgs' My question is twofold: How does readArgs work? How can that library be adjusted to allow it to work with a single argument as well? N.B. version 1.1 of ReadArgs

问题 With the hamlet templating language that comes with yesod, what is the best way of printing a comma-separated list? E.g. assume this code which just prints one entry after another, how do I insert commas in between the elements? Or maybe even add an “and” before the last entry: The values in the list are $ forall entry <- list #{entry} and that is it. Some templating languages such as Template Toolkit provide directives to detect the first or last iteration. 回答1: I don't think there's

问题 With the hamlet templating language that comes with yesod, what is the best way of printing a comma-separated list? E.g. assume this code which just prints one entry after another, how do I insert commas in between the elements? Or maybe even add an “and” before the last entry: The values in the list are $ forall entry <- list #{entry} and that is it. Some templating languages such as Template Toolkit provide directives to detect the first or last iteration. 回答1: I don't think there's

问题 I have following anonymous function: *Exercises> g = \(Sum n) -> Sum (n - 1) I use it like: *Exercises> g (Sum 56) Sum {getSum = 55} *Exercises> g 56 Sum {getSum = 55} The second example, how does the compiler convert 56 to Sum 56 ? In the prelude, I saw that Sum is an instance of Num , but it not clear about the conversion. 回答1: When Haskell sees an integer literal such as 56 , it interprets it as fromInteger 56 . The type of fromInteger is Num a => Integer -> a , so the type of this code is

问题 I have following anonymous function: *Exercises> g = \(Sum n) -> Sum (n - 1) I use it like: *Exercises> g (Sum 56) Sum {getSum = 55} *Exercises> g 56 Sum {getSum = 55} The second example, how does the compiler convert 56 to Sum 56 ? In the prelude, I saw that Sum is an instance of Num , but it not clear about the conversion. 回答1: When Haskell sees an integer literal such as 56 , it interprets it as fromInteger 56 . The type of fromInteger is Num a => Integer -> a , so the type of this code is

问题 I'm reading documentation on monad layers package and my brain is going to boil up. In the mmtl section of this document the author talks about invariant functor . It's method invmap is like fmap of Functor but it takes an inverse morphism (b -> a) also. I understand why author says that hoist of MFunctor is more powerful than tmap of Invariant but i don't see what's the point of that inverse morphism. Is there any example of an Invariant which can't be an instance of Functor ? 回答1: Here's a

问题 I'm trying to setup a new project using the nightly-2017-08-17 snapshot stack new test --resolver nightly-2017-08-17 However this gives the following error: Downloading template "new-template" to create project "test" in test/ ... Looking for .cabal or package.yaml files to use to init the project. Using cabal packages: - test/test.cabal Selected resolver: nightly-2017-08-17 Unable to parse cabal file: FromString "This package requires at least Cabal version 2.0" Nothing Cabal is in its

问题 I have a custom list type: data NNList a = Sing a | Append ( NNList a) ( NNList a) deriving (Eq) data CList a = Nil | NotNil ( NNList a) deriving (Eq) I'm trying to implement a function that returns the head and tail of a list: cListGet :: CList a -> Maybe (a, CList a) My attempt: cListGet :: CList a -> Maybe (a, CList a) cListGet Nil = Nothing cListGet xs@(NotNil nxs) = case nxs of Sing x -> (x, Nil) Append l r -> ((fst $ cListGet (NotNil l)), (Append (snd $ cListGet (NotNil l)), r)) Which

问题 I have a custom list type: data NNList a = Sing a | Append ( NNList a) ( NNList a) deriving (Eq) data CList a = Nil | NotNil ( NNList a) deriving (Eq) I'm trying to implement a function that returns the head and tail of a list: cListGet :: CList a -> Maybe (a, CList a) My attempt: cListGet :: CList a -> Maybe (a, CList a) cListGet Nil = Nothing cListGet xs@(NotNil nxs) = case nxs of Sing x -> (x, Nil) Append l r -> ((fst $ cListGet (NotNil l)), (Append (snd $ cListGet (NotNil l)), r)) Which

问题 I had a monadic computation. At some point it started requiring a MonadFail constraint because of a monadic pattern-match. My easy fix was to run it with this: fmap (either error id) . runErrorT Yet ouch: Deprecated: "Use Control.Monad.Trans.Except instead" So it appears that ErrorT is deprecated, and I'm to use ExceptT instead. That sounds fine from the outside, but it doesn't appear like ExceptT is a drop-in replacement at all! Just look at the instance declarations: instance (Monad m,