haskell

问题 I would like to install 'readline' using Cabal on Windows. When running the command it is saying that readline isn't found. Cabal is up to date. Command cabal install readline Output Resolving dependencies... Notice: installing into a sandbox located at C:\Users\Michael\Desktop\haskell-project\.cabal-sandbox Configuring readline-1.0.3.0... Failed to install readline-1.0.3.0 Build log ( C:\Users\Michael\Desktop\haskell-project\.cabal-sandbox\logs\readline-1.0.3.0.log ): Configuring readline-1

问题 I would like to install 'readline' using Cabal on Windows. When running the command it is saying that readline isn't found. Cabal is up to date. Command cabal install readline Output Resolving dependencies... Notice: installing into a sandbox located at C:\Users\Michael\Desktop\haskell-project\.cabal-sandbox Configuring readline-1.0.3.0... Failed to install readline-1.0.3.0 Build log ( C:\Users\Michael\Desktop\haskell-project\.cabal-sandbox\logs\readline-1.0.3.0.log ): Configuring readline-1

问题 I would like to install 'readline' using Cabal on Windows. When running the command it is saying that readline isn't found. Cabal is up to date. Command cabal install readline Output Resolving dependencies... Notice: installing into a sandbox located at C:\Users\Michael\Desktop\haskell-project\.cabal-sandbox Configuring readline-1.0.3.0... Failed to install readline-1.0.3.0 Build log ( C:\Users\Michael\Desktop\haskell-project\.cabal-sandbox\logs\readline-1.0.3.0.log ): Configuring readline-1

问题 I've got the following: asSqlBackendReader :: ReaderT SqlBackend m a -> ReaderT SqlBackend m a asSqlBackendReader = id insertEnt :: (Entity a) -> IO (Key (Entity a)) insertEnt x = runWithDb $ do insert $ x where runWithDb = runSqlite "test.db" . asSqlBackendReader The purpose of the asSqlBAckendReader is due to Persistent selectList causing error of "Couldn't match type ‘BaseBackend backend0’ with ‘SqlBackend’". I'm running into an error of: • Couldn't match type ‘PersistEntityBackend (Entity

问题 The first two Scrap Your Boilerplate papers describe a way of writing generic functions that work for general types, but have special cases for specific types. For instance, fromJSON from the aeson package, defines a generic function for converting from JSON, but provides special cases for types like list or Int : parseJSON :: (Data a) => Value -> Parser a parseJSON j = parseJSON_generic j `ext1R` list `ext1R` vector `ext2R'` mapAny `ext2R'` hashMapAny -- Use the standard encoding for all

问题 I've got the following: asSqlBackendReader :: ReaderT SqlBackend m a -> ReaderT SqlBackend m a asSqlBackendReader = id insertEnt :: (Entity a) -> IO (Key (Entity a)) insertEnt x = runWithDb $ do insert $ x where runWithDb = runSqlite "test.db" . asSqlBackendReader The purpose of the asSqlBAckendReader is due to Persistent selectList causing error of "Couldn't match type ‘BaseBackend backend0’ with ‘SqlBackend’". I'm running into an error of: • Couldn't match type ‘PersistEntityBackend (Entity

问题 I am having to write an expression to construct a Poly representation of the polynomial x^3 + x + 1. My algebraic data type Poly, which I wrote is: data Poly = Lit Integer | Var | Add Poly Poly | Mul Poly Poly The expression I can think of is this, but how can I be able to print out the result using print()?: expr::Poly->Poly expr = Add (Lit 1) $ Add (Var) $ Mul (Var) $ Mul Var Var Also, I'd like to write a function like this: showPoly::Poly->String showPoly (Lit x) = show x showPoly (Var) =

问题 The containers package already contains functions to convert Set and List to Map but it's missing one for Vector from vector package. Is this because package developers do not want to have dependencies on other (similar) packages? Shouldn't be some general package which converts between data structures? In this way I always have to convert to List to convert to other structures. But I want to minimize use because of performance. 来源： https://stackoverflow.com/questions/41960585/how-to-create

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