在Haskell中如何派生工作？
Haskell中的代数数据类型（ADT）可以通过派生它们自动变成某些类型类的实例（如Show ， Eq ）。
data Maybe a = Nothing | Just a
deriving (Eq, Ord)
我的问题是，这是如何deriving工作的，即Haskell如何知道如何实现派生类型类派生ADT的函数？
此外，为什么只能deriving到某些类型类？ 为什么我不能编写我自己可以派生的类型类？
简短的回答是，魔术:-)。 这就是说，自动派生被烘焙到Haskell规范中，并且每个编译器都可以选择以自己的方式实现它。 但是，如何使其可扩展性还有很多工作要做。
派生是Haskell的一个工具，可以让你编写自己的派生机制。
GHC用于提供一个名为Generic Classes的可扩展类型扩展，但它很少使用，因为它有点弱。 现在已经取消了，正在进行工作以整合本文所述的新的通用推导机制：http://www.dreixel.net/research/pdf/gdmh.pdf
有关更多信息，请参阅：
从Haskell 98报告：
允许派生实例的Prelude中的唯一类是Eq，Ord，Enum，Bounded，Show和Read ...
以下是如何派生这些类型类的说明：http://www.haskell.org/onlinereport/derived.html#derived-appendix
有可能使用模板Haskell生成实例声明，类似于派生子句。
下面的例子在Haskell Wiki中被无耻地窃取了：
在这个例子中，我们使用下面的Haskell代码
$(gen_render ''Body)
生成以下实例：
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分开放置）。
-- 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))
它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。 我们通过函数列表来生成实例的功能。
-- 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)
上述的帮助函数。
-- 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
还有一些从Syb III / replib 0.2中借用的辅助代码。
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