编辑:更新为自动派生AtoB
类。
这是一个似乎有效的解决方案。
没有 Monad 的通用相位映射
以下是预赛:
{-# LANGUAGE DataKinds, DeriveGeneric, FlexibleContexts,
FlexibleInstances, KindSignatures, MultiParamTypeClasses,
StandaloneDeriving, TypeFamilies, TypeFamilyDependencies,
TypeSynonymInstances, UndecidableInstances #-}
{-# OPTIONS_GHC -Wall #-}
import qualified GHC.Generics as GHC
import Generics.SOP
现在,假设我们有一个Phase
:
data Phase = A | B
和一个Selector
领域:
data Selector = Bar | Baz
想法是有一个类型类,它具有(1)一个关联的类型族,为每个可能的阶段提供与选择器关联的具体字段类型,以及(2)一个用于阶段之间映射的接口:
class IsField (sel :: Selector) where
type Field (p :: Phase) sel = r | r -> sel
fieldAtoB :: Field 'A sel -> Field 'B sel
给定一个包含Field
s 和非Field
s的通用实例的记录
data Foo p = Foo { bar :: Field p 'Bar
, baz :: Field p 'Baz
, num :: Int
} deriving (GHC.Generic)
deriving instance Show (Foo 'A)
deriving instance Show (Foo 'B)
instance Generic (Foo p)
和一个Foo 'A
值:
foo0 :: Foo 'A
foo0 = Foo (BarA ()) (BazA ()) 1
我们想定义一个通用的相位映射gAtoB
:
foo1 :: Foo 'B
foo1 = gAtoB foo0
它使用类型类中fieldAtoB
的每场相位图。IsField
关键步骤是定义一个单独的类型类AtoB
,专用于阶段A
到B
过渡,以充当IsField
类型类的桥梁。该AtoB
类型类将与机器结合使用,以逐个字段generics-sop
约束/匹配具体阶段A
和类型,并分派到适当的阶段映射函数。这是课程:B
fieldAtoB
class AtoB aty bty where
fieldAtoB' :: aty -> bty
幸运的是,可以为Field
s 自动派生实例,尽管它需要(大部分无害的)UndecidableInstances
扩展:
instance (IsField sel, Field 'A sel ~ aty, Field 'B sel ~ bty)
=> AtoB aty bty where
fieldAtoB' = fieldAtoB
我们可以为非Field
s 定义一个实例:
instance {-# OVERLAPPING #-} AtoB ty ty where
fieldAtoB' = id
请注意这里的一个限制——如果您在不同阶段定义具有相同具体类型的 a,则将使用Field
此重叠实例并将其忽略。fieldAtoB' = id
fieldAtoB
现在,对于一个特定的选择器Bar
,其底层类型应该是BarA
并且BarB
在各自的阶段中,我们可以定义以下IsField
实例:
-- Bar field
data BarA = BarA () deriving (Show) -- Field 'A 'Bar
data BarB = BarB () deriving (Show) -- Field 'B 'Bar
instance IsField 'Bar where
type Field 'A 'Bar = BarA -- defines the per-phase field types for 'Bar
type Field 'B 'Bar = BarB
fieldAtoB (BarA ()) = (BarB ()) -- defines the field phase map
我们可以为 提供类似的定义Baz
:
-- Baz field
data BazA = BazA () deriving (Show)
data BazB = BazB () deriving (Show)
instance IsField 'Baz where
type Field 'A 'Baz = BazA
type Field 'B 'Baz = BazB
fieldAtoB (BazA ()) = (BazB ())
现在,我们可以像这样定义通用gAtoB
转换:
gAtoB :: (Generic (rcrd 'A), Code (rcrd 'A) ~ xssA,
Generic (rcrd 'B), Code (rcrd 'B) ~ xssB,
AllZip2 AtoB xssA xssB)
=> rcrd 'A -> rcrd 'B
gAtoB = to . gAtoBS . from
where
gAtoBS :: (AllZip2 AtoB xssA xssB) => SOP I xssA -> SOP I xssB
gAtoBS (SOP (Z xs)) = SOP (Z (gAtoBP xs))
gAtoBS (SOP (S _)) = error "not implemented"
gAtoBP :: (AllZip AtoB xsA xsB) => NP I xsA -> NP I xsB
gAtoBP Nil = Nil
gAtoBP (I x :* xs) = I (fieldAtoB' x) :* gAtoBP xs
可能有一种方法可以使用generics-sop
组合器而不是这个明确的定义来做到这一点,但我无法弄清楚。
无论如何,根据上面的定义,它适用gAtoB
于记录,但它也适用于记录:Foo
foo1
Quux
data Quux p = Quux { bar2 :: Field p 'Bar
, num2 :: Int
} deriving (GHC.Generic)
deriving instance Show (Quux 'A)
deriving instance Show (Quux 'B)
instance Generic (Quux p)
quux0 :: Quux 'A
quux0 = Quux (BarA ()) 2
quux1 :: Quux 'B
quux1 = gAtoB quux0
main :: IO ()
main = do
print foo0
print foo1
print quux0
print quux1
请注意,我使用了带有Selector
数据类型的选择器,但是您可以将其重写为使用 type 的选择器(a :: Phase -> *)
,就像我在最后的示例中所做的那样。
Monad 上的通用阶段遍历
现在,您需要在IO
monad 上发生这种情况。这是一个修改后的版本:
{-# LANGUAGE DataKinds, DeriveGeneric, FlexibleContexts,
FlexibleInstances, KindSignatures, MultiParamTypeClasses,
StandaloneDeriving, TypeFamilies, TypeFamilyDependencies,
TypeSynonymInstances, UndecidableInstances #-}
{-# OPTIONS_GHC -Wall #-}
import qualified GHC.Generics as GHC
import Generics.SOP
import Control.Applicative
data Phase = A | B
data Selector = Bar | Baz
class IsField (sel :: Selector) where
type Field (p :: Phase) sel = r | r -> sel
fieldAtoB :: Field 'A sel -> IO (Field 'B sel)
data Foo p = Foo { bar :: Field p 'Bar
, baz :: Field p 'Baz
, num :: Int
} deriving (GHC.Generic)
deriving instance Show (Foo 'A)
deriving instance Show (Foo 'B)
instance Generic (Foo p)
foo0 :: Foo 'A
foo0 = Foo (BarA ()) (BazA ()) 1
foo1 :: IO (Foo 'B)
foo1 = gAtoB foo0
-- fieldAtoB :: Field 'A sel -> Field 'B sel
class AtoB aty bty where
fieldAtoB' :: aty -> IO bty
instance (IsField sel, Field 'A sel ~ aty, Field 'B sel ~ bty) => AtoB aty bty where
fieldAtoB' = fieldAtoB
instance {-# OVERLAPPING #-} AtoB ty ty where
fieldAtoB' = return
-- Bar field
data BarA = BarA () deriving (Show) -- Field 'A 'Bar
data BarB = BarB () deriving (Show) -- Field 'B 'Bar
instance IsField 'Bar where -- defines the per-phase field types for 'Bar
type Field 'A 'Bar = BarA
type Field 'B 'Bar = BarB
fieldAtoB (BarA ()) = return (BarB ()) -- defines the field phase map
-- Baz field
data BazA = BazA () deriving (Show)
data BazB = BazB () deriving (Show)
instance IsField 'Baz where
type Field 'A 'Baz = BazA
type Field 'B 'Baz = BazB
fieldAtoB (BazA ()) = return (BazB ())
gAtoB :: (Generic (rcrd 'A), Code (rcrd 'A) ~ xssA,
Generic (rcrd 'B), Code (rcrd 'B) ~ xssB,
AllZip2 AtoB xssA xssB)
=> rcrd 'A -> IO (rcrd 'B)
gAtoB r = to <$> (gAtoBS (from r))
where
gAtoBS :: (AllZip2 AtoB xssA xssB) => SOP I xssA -> IO (SOP I xssB)
gAtoBS (SOP (Z xs)) = SOP . Z <$> gAtoBP xs
gAtoBS (SOP (S _)) = error "not implemented"
gAtoBP :: (AllZip AtoB xsA xsB) => NP I xsA -> IO (NP I xsB)
gAtoBP Nil = return Nil
gAtoBP (I x :* xs) = I <$> fieldAtoB' x <**> pure (:*) <*> gAtoBP xs
data Quux p = Quux { bar2 :: Field p 'Bar
, num2 :: Int
} deriving (GHC.Generic)
deriving instance Show (Quux 'A)
deriving instance Show (Quux 'B)
instance Generic (Quux p)
quux0 :: Quux 'A
quux0 = Quux (BarA ()) 2
quux1 :: IO (Quux 'B)
quux1 = gAtoB quux0
main :: IO ()
main = do
print foo0
foo1val <- foo1
print foo1val
print quux0
quux1val <- quux1
print quux1val
适应您的问题
这是一个重写的版本,以尽可能接近您的原始设计。同样一个关键限制是,将Config
使用具有相同配置时间和运行时间类型的a,而不是在其实例toRunTime' = return
中给出的任何其他定义。Config
{-# LANGUAGE DataKinds, DeriveGeneric, FlexibleContexts,
FlexibleInstances, KindSignatures, MultiParamTypeClasses,
StandaloneDeriving, TypeFamilies, TypeFamilyDependencies,
TypeSynonymInstances, UndecidableInstances #-}
{-# OPTIONS_GHC -Wall #-}
import qualified GHC.Generics as GHC
import Generics.SOP
import Control.Applicative
data UsagePhase = ConfigTime | RunTime
class Config (sel :: UsagePhase -> *) where
type Phase (p :: UsagePhase) sel = r | r -> sel
toRunTime :: Phase 'ConfigTime sel -> IO (Phase 'RunTime sel)
class ConfigRun cty rty where
toRunTime' :: cty -> IO rty
instance (Config (sel :: UsagePhase -> *),
Phase 'ConfigTime sel ~ cty,
Phase 'RunTime sel ~ rty) => ConfigRun cty rty where
toRunTime' = toRunTime
instance {-# OVERLAPPING #-} ConfigRun ty ty where
toRunTime' = return
-- DatabaseConfig field
data DatabaseConfig (p :: UsagePhase)
data ConnectInfo = ConnectInfo () deriving (Show)
data ConnectionPool = ConnectionPool () deriving (Show)
instance Config DatabaseConfig where
type Phase 'ConfigTime DatabaseConfig = ConnectInfo
type Phase 'RunTime DatabaseConfig = ConnectionPool
toRunTime (ConnectInfo ()) = return (ConnectionPool ())
-- KinesisConfig field
data KinesisConfig (p :: UsagePhase)
data KinesisInfo = KinesisInfo () deriving (Show)
data KinesisStream = KinesisStream () deriving (Show)
instance Config KinesisConfig where
type Phase 'ConfigTime KinesisConfig = KinesisInfo
type Phase 'RunTime KinesisConfig = KinesisStream
toRunTime (KinesisInfo ()) = return (KinesisStream ())
-- CfgMyHostName field
data CfgMyHostName = CfgMyHostName String deriving (Show)
data UiServerConfig (p :: UsagePhase) = CfgUiServerC
{ userDatabase :: Phase p DatabaseConfig
, cmsDatabase :: Phase p DatabaseConfig
, kinesisStream :: Phase p KinesisConfig
, myHostName :: CfgMyHostName
, myPort :: Int
} deriving (GHC.Generic)
deriving instance Show (UiServerConfig 'ConfigTime)
deriving instance Show (UiServerConfig 'RunTime)
instance Generic (UiServerConfig p)
gToRunTime :: (Generic (rcrd 'ConfigTime), Code (rcrd 'ConfigTime) ~ xssA,
Generic (rcrd 'RunTime), Code (rcrd 'RunTime) ~ xssB,
AllZip2 ConfigRun xssA xssB)
=> rcrd 'ConfigTime -> IO (rcrd 'RunTime)
gToRunTime r = to <$> (gToRunTimeS (from r))
where
gToRunTimeS :: (AllZip2 ConfigRun xssA xssB) => SOP I xssA -> IO (SOP I xssB)
gToRunTimeS (SOP (Z xs)) = SOP . Z <$> gToRunTimeP xs
gToRunTimeS (SOP (S _)) = error "not implemented"
gToRunTimeP :: (AllZip ConfigRun xsA xsB) => NP I xsA -> IO (NP I xsB)
gToRunTimeP Nil = return Nil
gToRunTimeP (I x :* xs) = I <$> toRunTime' x <**> pure (:*) <*> gToRunTimeP xs
cfg0 :: UiServerConfig 'ConfigTime
cfg0 = CfgUiServerC (ConnectInfo ()) (ConnectInfo ()) (KinesisInfo())
(CfgMyHostName "localhost") 10
main :: IO ()
main = do
print cfg0
run0 <- gToRunTime cfg0
print run0