使用GHC泛型定义“空"样功能?

编程入门行业动态更新时间:2024-10-04 17:32:11

本文介绍了使用GHC泛型定义“空"样功能?的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！问题描述

我正在为Zoho REST API编写一个客户端库，并且有一堆不同的记录类型，这些记录类型具有所有也许是字段，即:

I am writing a client library for the Zoho REST API and have a bunch of different record types that have all Maybe a fields, i.e:

data Approval = Approval { apDelegate :: Maybe Bool , apApprove :: Maybe Bool , apReject :: Maybe Bool , apResubmit :: Maybe Bool } deriving (Eq, Show, Generic) data ContactSpecialFields = ContactSpecialFields { csfCurrencySymbol :: Maybe Text -- $currency_symbol , csfState :: Maybe Text -- $state , csfProcessFlow :: Maybe Bool -- $process_flow , csfApproved :: Maybe Bool -- $approved , csfApproval :: Approval -- $approval , csfEditable :: Maybe Bool -- $editable } deriving (Eq, Show) -- and so on

我需要一种定义此类空"记录的方法，例如:

I need an way to define "empty" records of such types, eg:

emptyApproval :: Approval emptyApproval = Approval { apDelegate = Nothing , apApprove = Nothing , apReject = Nothing , apResubmit = Nothing }

因此，我联系了 GHC.Generics ，并得到了一些有用的信息(这是越野车！):

Therefore, I reached out for GHC.Generics and got something working (which is buggy!):

-- These parts seem logically correct to me... class GEmptyZohoStructure f where gEmptyZohoStructure :: f p instance (GEmptyZohoStructure f, GEmptyZohoStructure g) => GEmptyZohoStructure (f :*: g) where gEmptyZohoStructure = (gEmptyZohoStructure :: f p) :*: (gEmptyZohoStructure :: g p) instance GEmptyZohoStructure Maybe where gEmptyZohoStructure = Nothing class EmptyZohoStructure a where emptyZohoStructure :: a default emptyZohoStructure :: (Generic a, (GEmptyZohoStructure (Rep a))) => a emptyZohoStructure = GHC.Generics.to gEmptyZohoStructure -- Whereas these parts are random type-class instances that I've written, just -- to get the code to compile. instance (GEmptyZohoStructure f) => GEmptyZohoStructure (M1 i t f) where gEmptyZohoStructure = (gEmptyZohoStructure :: f p) instance (GEmptyZohoStructure f) => GEmptyZohoStructure (K1 i (f p)) where gEmptyZohoStructure = gEmptyZohoStructure instance EmptyZohoStructure Approval

在编译代码时，以下(可理解的)结果会在运行时导致堆栈溢出:

while the code compiles, the following (understandably) results in a stack overflow during runtime:

ghci> emptyZohoStructure :: Approval *** Exception: stack overflow

我一直在 www.stackage/haddock/lts-12.1/base-4.11.1.0/GHC-Generics.html#g:12 ，应在适当位置提交对于传递给 encode 函数的参数，它提供了一个机会来解包 M1 / K1 构造函数并建立一些有意义的递归层次结构.如何为用例(其中泛型函数实际上没有任何参数)编写 M1 和 K1 的泛型?

I was following the encode tutorial given at www.stackage/haddock/lts-12.1/base-4.11.1.0/GHC-Generics.html#g:12 , where due to the argument being passed to the encode function, it allows one the opportunity to unwrap the M1 / K1 constructor and build some meaningful recursion hierarchy. How do I write generic for M1 and K1 for my use-case (where the generic function doesn't really have any arguments)?

推荐答案

在泛型类型类中定义 GEmptyZohoStructure Maybe 毫无意义.

There is no point defining GEmptyZohoStructure Maybe in a typeclass for generics.

class G f where gempty' :: f p instance (G f, G g) => G ( f :*: g) where gempty' = gempty' :*: gempty' instance G c => G (D1 x c) where gempty' = M1 gempty' instance G s => G (C1 x s) where gempty' = M1 gempty' instance E t => G (S1 m (Rec0 t)) where -- the key instance gempty' = M1 (K1 gempty) class E a where gempty :: a default gempty :: (Generic a, G (Rep a)) => a gempty = to gempty' instance E (Maybe a) where gempty = Nothing