ghc

问题 In this example, action is an infinite loop created by mistake. Is there a way to detect such loops in a GHC program? action bucket manager url = catch (action bucket manager url) (\(e :: HttpException) -> Logger.warn $ "Problems with " ++ url) 回答1: Short answer: no. It certainly isn't possible to notice every infinite loop one could write down; this is famously known as the halting problem and the formal proof that one cannot write a loop-detecting program is so famous that there's even a Dr

问题 Consider the following two statements: (a `par` b) `pseq` (a + b) and a `par` (b `pseq` (a + b)) Can someone explain how their behavior differ from each other? For the first one, if the main thread has done with computing b but the spark computing a hasn't finished, will the main thread proceed to compute a + b ? 回答1: par a b is semantically equivalent to b , but it gives the hint that it might be useful to start evaluating a early. On the otherhand pseq forces the evaluation of its first

问题 The following code crashes when using GHC on Windows. It works perfectly on Linux. Does this make any sense or is there a bug? module Main where import qualified Text.Regex as Re -- from regex-compat import Debug.Trace main :: IO () main = do putStr $ cleanWord "jan" putStr $ cleanWord "dec" putStr $ cleanWord "jun" -- crashes here cleanWord :: String -> String cleanWord word_ = let word = trace (show word_) word_ in let re = Re.mkRegexWithOpts "(jun|jul|aug|sep|oct|nov|dec)" True False in Re

问题 I'm playing around with the ConstraintKinds extension of GHC. I have the following data type, which is just a box for things fulfilling some one parameter constraint c : data Some (c :: * -> Constraint) where Some :: forall a. c a => a -> Some c For example, I could construct a box with some kind of number (arguably not very useful). x :: Some Num x = Some (1 :: Int) Now, as long as c includes the constraint Show , I could provide an instance of Show (Some c) . instance ??? => Show (Some c)

问题 Consider the simple definition of a length-indexed vector: data Nat = Z | S Nat infixr 5 :> data Vec (n :: Nat) a where V0 :: Vec 'Z a (:>) :: a -> Vec n a -> Vec ('S n) a Naturally I would at some point need the following function: vec2list :: Vec n a -> [a] However, this function is really just a fancy identity. I believe that the runtime representations of these two types are the same, so vec2list :: Vec n a -> [a] vec2list = unsafeCoerce should work. Alas, it does not: >vec2list ('a' :>

问题 I am trying to import Writer monad which is defined in mtl-2.2.2 package which I have installed as I checked with ghc-pkg list . When I tried to do import Control.Monad.Writer in ghci it is giving me an error. I don't understand why? Prelude> import Control.Monad.Writer <no location info>: error: Could not find module ‘Control.Monad.Writer’ Perhaps you meant Control.Monad.Fail (from base-4.11.1.0) Control.Monad.Fix (from base-4.11.1.0) Control.Monad.Zip (from base-4.11.1.0) 来源： https:/

问题 suppose I have two modules NecessaryModule1 & NecessaryModule2 (as outlined in the post Haskell : loading ALL files in current directory path. Then I have noticed in both WinGHCi and GHCi that if I do : > :load NecessaryModule1 [1 of 1] Compiling NecessaryModule1 ( NecessaryModule1.hs, interpreted ) Ok, modules loaded: NecessaryModule1. > addNumber1 2 3 5 > :load NecessaryModule2 [1 of 1] Compiling NecessaryModule2 ( NecessaryModule2.hs, interpreted ) Ok, modules loaded: NecessaryModule2. >

问题 I need glib for threadscope to work. But I am not able to install it using cabal. I tried cabal install glib but getting the following error. I even tried downloading the library and installing it locally but get the same error. I am using Cygwin on Windows. I ran Cygwin setup to make sure I have the missing libraries. I was able to find glib, gobject but not gthread. Gtk2HsSetup.hs:25:2: warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL

问题 I need glib for threadscope to work. But I am not able to install it using cabal. I tried cabal install glib but getting the following error. I even tried downloading the library and installing it locally but get the same error. I am using Cygwin on Windows. I ran Cygwin setup to make sure I have the missing libraries. I was able to find glib, gobject but not gthread. Gtk2HsSetup.hs:25:2: warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL

问题 I wanted to use the Control.Monad.Except module, but it turned out I had an outdated mtl package (It caused an import error - I had an obsolete module Control.Monad.Error ). So I did sudo cabal install mtl And it installed the 2.2.2 version. However, now I had two versions installed, 2.1.2 and 2.2.2 which still caused an import error. I followed instructions here and did sudo ghc-pkg unregister --force mtl-2.1.2 to remove the old version. But now I get an error: Could not find module `Control