在Haskell中派生如何工作？

Question

Haskell中的代数数据类型 （ADT）可以通过派生而自动成为某些类型类 （例如Show ， Eq ）的实例。

data  Maybe a  =  Nothing | Just a
  deriving (Eq, Ord)

我的问题是，此deriving如何工作，即Haskell如何知道如何为派生的ADT实现派生的类型类的功能？

另外，为什么deriving仅限于某些类型类？ 为什么我不能编写自己的可以派生的类型类？

Answer 1

简短的答案是，魔术:-)。 也就是说，自动派生已纳入Haskell规范，并且每个编译器都可以选择以自己的方式实现它。 但是，如何使它可扩展有很多工作。

Derive是Haskell允许您编写自己的派生机制的工具。

GHC过去提供了称为Generic Classes的可派生类型类扩展，但由于它有些弱，因此很少使用。 现在已经将其删除，并且正在进行整合本文所述的新的通用派生机制的工作： http : //www.dreixel.net/research/pdf/gdmh.pdf

有关更多信息，请参见：

• GHC Wiki： http : //hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/GenericDeriving
• Haskell Wiki： http ： //www.haskell.org/haskellwiki/Generics
• 骇客： http ://hackage.haskell.org/package/generic-deriving

Answer 2

从Haskell 98报告中：

Prelude中唯一允许派生实例的类是Eq，Ord，Enum，Bounded，Show和Read ...

这是如何派生这些类型类的描述： http : //www.haskell.org/onlinereport/derived.html#derived-appendix

Answer 3

可以使用模板Haskell以与派生子句类似的方式生成实例声明。

以下示例是从Haskell Wiki偷偷窃取的：

在此示例中，我们使用以下Haskell代码

 $(gen_render ''Body) 

产生以下实例：

 instance TH_Render Body where render (NormalB exp) = build 'normalB exp render (GuardedB guards) = build 'guardedB guards 

上面的函数gen_render定义如下。 （请注意，此代码必须与上述用法位于单独的模块中）。

 -- Generate an intance of the class TH_Render for the type typName gen_render :: Name -> Q [Dec] gen_render typName = do (TyConI d) <- reify typName -- Get all the information on the type (type_name,_,_,constructors) <- typeInfo (return d) -- extract name and constructors i_dec <- gen_instance (mkName "TH_Render") (conT type_name) constructors -- generation function for method "render" [(mkName "render", gen_render)] return [i_dec] -- return the instance declaration -- function to generation the function body for a particular function -- and constructor where gen_render (conName, components) vars -- function name is based on constructor name = let funcName = makeName $ unCapalize $ nameBase conName -- choose the correct builder function headFunc = case vars of [] -> "func_out" otherwise -> "build" -- build 'funcName parm1 parm2 parm3 ... in appsE $ (varE $ mkName headFunc):funcName:vars -- put it all together -- equivalent to 'funcStr where funcStr CONTAINS the name to be returned makeName funcStr = (appE (varE (mkName "mkName")) (litE $ StringL funcStr)) 

其中使用以下功能和类型。

首先，一些类型的同义词使代码更具可读性。

 type Constructor = (Name, [(Maybe Name, Type)]) -- the list of constructors type Cons_vars = [ExpQ] -- A list of variables that bind in the constructor type Function_body = ExpQ type Gen_func = Constructor -> Cons_vars -> Function_body type Func_name = Name -- The name of the instance function we will be creating -- For each function in the instance we provide a generator function -- to generate the function body (the body is generated for each constructor) type Funcs = [(Func_name, Gen_func)] 

主要的可重用功能。 我们向其传递函数列表以生成实例的函数。

 -- construct an instance of class class_name for type for_type -- funcs is a list of instance method names with a corresponding -- function to build the method body gen_instance :: Name -> TypeQ -> [Constructor] -> Funcs -> DecQ gen_instance class_name for_type constructors funcs = instanceD (cxt []) (appT (conT class_name) for_type) (map func_def funcs) where func_def (func_name, gen_func) = funD func_name -- method name -- generate function body for each constructor (map (gen_clause gen_func) constructors) 

以上的辅助功能。

 -- Generate the pattern match and function body for a given method and -- a given constructor. func_body is a function that generations the -- function body gen_clause :: (Constructor -> [ExpQ] -> ExpQ) -> Constructor -> ClauseQ gen_clause func_body data_con@(con_name, components) = -- create a parameter for each component of the constructor do vars <- mapM var components -- function (unnamed) that pattern matches the constructor -- mapping each component to a value. (clause [(conP con_name (map varP vars))] (normalB (func_body data_con (map varE vars))) []) -- create a unique name for each component. where var (_, typ) = newName $ case typ of (ConT name) -> toL $ nameBase name otherwise -> "parm" where toL (x:y) = (toLower x):y unCapalize :: [Char] -> [Char] unCapalize (x:y) = (toLower x):y 

还有一些借鉴了Syb III / replib 0.2的帮助程序代码。

 typeInfo :: DecQ -> Q (Name, [Name], [(Name, Int)], [(Name, [(Maybe Name, Type)])]) typeInfo m = do d <- m case d of d@(DataD _ _ _ _ _) -> return $ (simpleName $ name d, paramsA d, consA d, termsA d) d@(NewtypeD _ _ _ _ _) -> return $ (simpleName $ name d, paramsA d, consA d, termsA d) _ -> error ("derive: not a data type declaration: " ++ show d) where consA (DataD _ _ _ cs _) = map conA cs consA (NewtypeD _ _ _ c _) = [ conA c ] {- This part no longer works on 7.6.3 paramsA (DataD _ _ ps _ _) = ps paramsA (NewtypeD _ _ ps _ _) = ps -} -- Use this on more recent GHC rather than the above paramsA (DataD _ _ ps _ _) = map nameFromTyVar ps paramsA (NewtypeD _ _ ps _ _) = map nameFromTyVar ps nameFromTyVar (PlainTV a) = a nameFromTyVar (KindedTV a _) = a termsA (DataD _ _ _ cs _) = map termA cs termsA (NewtypeD _ _ _ c _) = [ termA c ] termA (NormalC c xs) = (c, map (\\x -> (Nothing, snd x)) xs) termA (RecC c xs) = (c, map (\\(n, _, t) -> (Just $ simpleName n, t)) xs) termA (InfixC t1 c t2) = (c, [(Nothing, snd t1), (Nothing, snd t2)]) conA (NormalC c xs) = (simpleName c, length xs) conA (RecC c xs) = (simpleName c, length xs) conA (InfixC _ c _) = (simpleName c, 2) name (DataD _ n _ _ _) = n name (NewtypeD _ n _ _ _) = n name d = error $ show d simpleName :: Name -> Name simpleName nm = let s = nameBase nm in case dropWhile (/=':') s of [] -> mkName s _:[] -> mkName s _:t -> mkName t