GHC has a few language flags, such as DeriveFunctor , DeriveDataTypeable etc., which enable compiler generation of derived instances for type classes other than those allowed in Haskell 98. This especially makes sense for something like Functor , where the laws of that class dictate an obvious, "natural" derived instance. So why not for Monoid ? It seems like for any data type with a
GHC有一些语言标志，例如DeriveFunctor ， DeriveDataTypeable等，它们可以编译器生成除Haskell 98中允许的类型类的派生实例。这对于类似Functor东西特别Functor ，其中该类的法律规定明显的“自然”派生实例。 那么为什么不为Monoid ？ 对于具有单个数据构造函数的任何数据类型来说似乎如此： data T = MkT a b c ... 人们可以机械地产生一个Monoid实例（借口伪代码）： instance (Monoid a, Monoid b, Monoid c, ...) =>
The Haskell Wiki does a good job of explaining how to use existential types, but I don't quite grok the theory behind them. Consider this example of an existential type: data S = forall a. Show a => S a -- (1) to define a type wrapper for things that we can convert to a String . The wiki mentions that what we really want to define is a type like data S = S (exists a. Show a =>
Haskell Wiki在解释如何使用存在类型方面做得很好，但我并不完全赞同它们背后的理论。 考虑这种存在类型的例子： data S = forall a. Show a => S a -- (1) 为我们可以转换为String东西定义一个类型包装器。 维基提到，我们真正想要定义的是类似的 data S = S (exists a. Show a => a) -- (2) 即一个真正的“存在”类型 - 我认为这是一个真正的“存在”类型：“数据构造函数S采用Show实例存在并包装它的任何类型
我写这个有什么不同？ data Book = Book Int Int newtype Book = Book(Int, Int) -- "Book Int Int" is syntactically invalid Great question! There are several key differences. Representation A newtype guarantees that your data will have exactly the same representation at runtime, as the type that you wrap. While data declares a brand new data structure at runtime. So the key point here is th
我写这个有什么不同？ data Book = Book Int Int newtype Book = Book(Int, Int) -- "Book Int Int" is syntactically invalid 伟大的问题！ 有几个关键的区别。 表示 一个newtype保证您的数据将有完全相同的表示在运行时，为你包的类型。 data在运行时声明一个全新的数据结构。 因此，这里的关键点是，对于构建newtype保证在编译时被删除。 例子： data Book = Book Int Int newtype Book = Book (Int, Int)
Just started learning Haskell and Im trying to implement a max function to find the max of a list recursively max' :: (Num b) => [a] -> b max' [] = 0 max' (x:xs) | x > max' xs = x | otherwise = max' xs but am getting the error when trying to compile Couldn't match expected type 'b' with actual type 'a' 'a' is a rigid type variable bound by the type
刚开始学习Haskell和Im试图实现一个最大函数来递归地查找列表的最大值 max' :: (Num b) => [a] -> b max' [] = 0 max' (x:xs) | x > max' xs = x | otherwise = max' xs 但在编译时遇到错误 无法与实际类型'a'匹配预期类型'b''a'是由类型签名绑定的刚性类型变量：max':: forall b a。 （数字b，数字a）=> [a] - > b在实现函数.hs：5：1-34'b'是一个刚性类型变
I'm trying learning Haskell and I ended up reading this and I'm trying to figure out what some parts of the code do. At first, there's a data type definition: infixl 4 :+: infixl 5 :*:, :/: infixr 6 :^: data Expr a = Var Char | Const a | (Expr a) :+: (Expr a) | (Expr a) :*: (Expr a) | (Expr a) :^: (Expr a) | (Expr
我正在学习Haskell，并最终读到了这个，我试图找出代码的某些部分的作用。 首先，有一个数据类型定义： infixl 4 :+: infixl 5 :*:, :/: infixr 6 :^: data Expr a = Var Char | Const a | (Expr a) :+: (Expr a) | (Expr a) :*: (Expr a) | (Expr a) :^: (Expr a) | (Expr a) :/: (Expr a) deriving (Show, Eq) 接下来有一个功能： simp
I was reading a lecture note on Haskell when I came across this paragraph (http://www.seas.upenn.edu/~cis194/lectures/02-lists.html): This “not caring” is what the “parametric” in parametric polymorphism means. All Haskell functions must be parametric in their type parameters; the functions must not care or make decisions based on the choices for these parameters. A function can't do one
我在阅读本段（http://www.seas.upenn.edu/~cis194/lectures/02-lists.html）时正在阅读关于Haskell的讲稿： 这种“不关心”是参数多态性中“参数化”的含义。 所有的Haskell函数都必须是参数类型参数; 基于这些参数的选择，功能不必关心或做出决定。 当a是一个函数时，函数不能做一件事，而当a是Bool时，函数不能做一件事。 Haskell根本没有提供写这种操作的工具。 这种语言属性称为参数性。 参数化有许多深刻和深刻的后果
This question already has an answer here: Memory footprint of Haskell data types 2 answers The GHC documentation has some good information. But basically, you're correct in saying that an Int value is a pointer to a thunk. However, an unboxed value is not a pointer to the unboxed value, it is the unboxed value itself. Also, the Haskell standard report merely gives the lower limit on th
这个问题在这里已经有了答案： Haskell数据类型的内存占用情况2个答案 GHC文件有一些很好的信息。 但基本上，你说的Int值是指向thunk的指针是正确的。 但是，unboxed值不是指向unboxed值的指针，而是unboxed值本身。 此外，Haskell标准报告仅给出Int范围的下限。 IIRC，GHC Int有超过30位。 我不认为GHC会使用额外的非装箱类型的位来存储任何元数据，但它确实使用了指针位来执行此操作。 请参阅此页面了解更多详情。
I'm wondering why this piece of code doesn't type-check: {-# LANGUAGE ScopedTypeVariables, Rank2Types, RankNTypes #-} {-# OPTIONS -fglasgow-exts #-} module Main where foo :: [forall a. a] foo = [1] ghc complains: Could not deduce (Num a) from the context () arising from the literal `1' at exist5.hs:7:7 Given that: Prelude> :t 1 1 :: (Num t) => t Prelude> it seems that
我想知道为什么这段代码没有进行类型检查： {-# LANGUAGE ScopedTypeVariables, Rank2Types, RankNTypes #-} {-# OPTIONS -fglasgow-exts #-} module Main where foo :: [forall a. a] foo = [1] ghc抱怨： Could not deduce (Num a) from the context () arising from the literal `1' at exist5.hs:7:7 鉴于： Prelude> :t 1 1 :: (Num t) => t Prelude> 似乎（Num t）上下文不能匹配arg的（）上下文。
When I compile a Haskell file with ghci , typically with :load , and if there is no type error, all the expressions are loaded in the ghc interpreter. It's very nice: I can play around with :t to figure out the type of various expressions. My problem is: if there is a tiny error somewhere, ghci is not able to load anything (not even the imported modules!!), which makes finding the right ty
当我使用ghci编译Haskell文件时，通常使用:load ，并且如果没有类型错误，则所有表达式都将加载到ghc解释器中。 这是非常好的：我可以玩弄:t找出各种表情的类型。 我的问题是：如果在某个地方有一个小错误，ghci无法加载任何东西（甚至不是导入的模块!!），这使得找到合适的类型更加困难。 我总是做同样的：注释掉不进行类型检查，发现室内用相关类型的所有位:t在ghci中，和去评论。 但这太乏味了！ 是否有更好的“部分编
I'm trying to build a standalone program for the first time in Haskell, and am having trouble figuring out how to get ghc --make working with the directory organization of my liking. As the moment I have the following tree: readme.md src/ Main.hs Ciphers/ Affine.hs Shift.hs Substitution.hs Tests/ HCTests.hs With the following imports: Main.hs: module Main where
我试图在Haskell中第一次构建一个独立程序，并且无法弄清楚如何获得ghc - 与我所喜欢的目录组织一起工作。 正如我有以下树的那一刻： readme.md src/ Main.hs Ciphers/ Affine.hs Shift.hs Substitution.hs Tests/ HCTests.hs 通过以下导入： Main.hs： module Main where import Tests.HCTests import Ciphers.Affine import Ciphers.Shift import Ciphers.Substitution HCTests.hs module