ghc

问题 Why cannot top level module be set to "Main" in Hint (Language.Haskell.Interpreter)? Allow me to demonstrate: module Main where import Language.Haskell.Interpreter import Control.Monad main = do res <- runInterpreter (test "test") case res of Left e -> putStrLn (show e) Right t -> putStrLn (show t) return () test :: String -> Interpreter () test mname = do loadModules [mname ++ ".hs"] setTopLevelModules ["Main"] Will result in: NotAllowed "These modules are not interpreted:\nMain\n" 回答1: As

问题 I'm trying to write a function which allows you to "apply" a list of Strings to an arbitrary function. Here's what I've got so far: {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, OverlappingInstances, TypeFamilies #-} class Apply a c where apply :: a -> [String] -> c instance (Read a, Apply b c) => Apply (a -> b) c where apply _ [] = error "not enough arguments" apply f (x:xs) = apply (f (read x)) xs instance (a ~ b) => Apply a b where apply f [] = f apply _ (_:_) = error "too many

问题 I'm trying to learn about how to use GHC.Generics. A fascinating topic but daunting. While reading through the blog entry 24 Days of GHC Extensions: DeriveGeneric, I learned how to take a value and navigate its Rep . Okay. However, reading the blog entry Building data constructors with GHC Generics which describes the analog of constructing the Rep and converting it back to a value, I got stumped. I've read through a number of other resources, but to no great help. In the blog entry is the

问题 Suppose that we have a typeclass class (A a, B a) => C a where . Using newtype will allow us to clone a data type and then automatically derive the instances via the GeneralizedNewtypeDeriving language extension (See how to write a derivable class? and Handling multiple types with the same internal representation and minimal boilerplate?). QUESTION : Is it possible to get ghc to automatically derive A and C , but to use our own specified implementation of B in deriving C ? For example the

问题 So, I have an AST data type with a large number of cases, which is parameterized by an "annotation" type data Expr a = Plus a Int Int | ... | Times a Int Int I have annotation types S and T , and some function f :: S -> T . I want to take an Expr S and convert it to an Expr T using my conversion f on each S which occurs within an Expr value. Is there a way to do this using SYB or generics and avoid having to pattern match on every case? It seems like the type of thing that this is suited for.

问题 I wrote a small program, kind of specialized HTTP server in haskell, which is not much more complex than the code below. What puzzles me is its memory consumption. Say, when I run a test compiled from the enclosed code and make several POST requests containing up to 20Mb body whole program will have VM size of ~800Mb and this sounds odd. And this space is not returned to system if I leave an instance of such program running idle. What does this mean? import System.IO import Network.HTTP

问题 Consider the following examples ( λ> = ghci, $ = shell): λ> writeFile "d" $ show "d" $ cat d "d" λ> writeFile "d" "d" $ cat d d λ> writeFile "backslash" $ show "\\" $ cat backslash "\\" λ> writeFile "backslash" "\\" $ cat backslash \ λ> writeFile "cat" $ show "🐈" -- U+1F408 $ cat cat "\128008" λ> writeFile "cat" "🐈" $ cat cat 🐈 I understand that another way of "\128008" is just another way of representing "🐈" in Haskell source code. My question is: why does the "🐈" example behave like the

问题 I currently try to solve my problem HXT: Can an input change with the arrow syntax? and therefore and want to see the haskell code after the ghc compiler desugars the Arrow syntax. How can I do this? I already tried -ddump-ds but with this flag I get a horrible long code because also all types are resolved. Is there a way to see the code with just arrow syntax desugaring? 回答1: The original arrow project provided a parser, called arrowp , which is available on Hackage and translates the arrow

问题 I'm on the haskell platform, GHC 6.12.1 as apt-get installed on Debian Squeeze. How can I get the underlying errno out of an IOError, given that I need it on a different thread to where it was originally raised? The reason I need this is because I'm implementing a network protocol which exposes the actual errno value on the wire. Do I need to reconstruct it? 回答1: errno is thread-local in GHC. You'll need to trap the errno in one thread; then send the value down a Chan or other communication

问题 Title pretty much says it all. I'm looking for something like this: f :: Int -> Bool -> Int f = _body Djinn can use theorem proving to generate code for such a function by proving that the type is inhabited. I'm wondering, is there an existing way to get this functionality from within Emacs? So instead of writing TemplateHaskell in my code, I just run a command on my code and it inserts the generated code? I have ghc-mod installed, but I'm not very familiar with it. 回答1: Quoting the relevant