[英]Defining an “mempty”-like function with GHC Generics?
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, ie: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,这些记录类型都包含所有Maybe a
字段,即:
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
I need an way to define "empty" records of such types, eg:我需要一种方法来定义此类类型的“空”记录,例如:
emptyApproval :: Approval
emptyApproval = Approval
{ apDelegate = Nothing
, apApprove = Nothing
, apReject = Nothing
, apResubmit = Nothing
}
Therefore, I reached out for GHC.Generics
and got something working (which is buggy!):因此,我联系了GHC.Generics
并得到了一些工作(这是有问题的!):
-- 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
while the code compiles, the following (understandably) results in a stack overflow during runtime:在代码编译时,以下(可以理解)会导致运行时堆栈溢出:
ghci> emptyZohoStructure :: Approval
*** Exception: stack overflow
I was following the encode
tutorial given at https://www.stackage.org/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.我正在关注https://www.stackage.org/haddock/lts-12.1/base-4.11.1.0/GHC-Generics.html#g:12给出的encode
教程,其中由于参数被传递给encode
函数,它允许人们有机会解开M1
/ K1
构造函数并构建一些有意义的递归层次结构。 How do I write generic for M1
and K1
for my use-case (where the generic function doesn't really have any arguments)?我如何为我的用例编写M1
和K1
的泛型(泛型函数实际上没有任何参数)?
There is no point defining GEmptyZohoStructure Maybe
in a typeclass for generics.在泛型的类型类中定义GEmptyZohoStructure Maybe
毫无意义。
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
After that you can define any product type consisting of Maybe values.之后,您可以定义由 Maybe 值组成的任何产品类型。
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