我定义了以下类:
class Configuration c where
input :: (PortIn pin, Show pin) => c -> pin
startSt :: (SysState st, Show st) => c -> st
class (Eq pin, Show pin) => PortIn pin
class (Eq st, Show st) => SysState st
以下类型和实例:
--inputs
data PIn = PIn { _clk :: Bit
, _reset :: Bool
, _start :: Bool
, _stop :: Bool
} deriving (Eq)
instance PortIn PIn
instance Show PIn where
show PIn {..} =
"PIn\n\t _clk = " P.++ show _clk
P.++ "\n\t _reset = " P.++ show _reset
P.++ "\n\t _start = " P.++ show _start
P.++ "\n\t _stop = " P.++ show _stop
--Outputs and state data
data St = St { _cnt_en :: Bool
, _count_us :: BitVector 4
, _stop_d1 :: Bool
, _stop_d2 :: Bool
, _count :: BitVector 4
} deriving (Eq)
instance SysState St
instance Show St where
show St {..} =
"St\n\t _cnt_en = " P.++ show _cnt_en
P.++ "\n\t _count_us = " P.++ show _count_us
P.++ "\n\t _stop_d1 = " P.++ show _stop_d1
P.++ "\n\t _stop_d2 = " P.++ show _stop_d2
P.++ "\n\t _count = " P.++ show _count
为什么我无法在此处创建 Configuration 实例:
data Config = Config { input' :: PIn
, startSt' :: St
} deriving (Eq)
instance Show Config where
show Config {..} =
"Config:\n input = " P.++ show input'
P.++ "\n startSt = " P.++ show startSt'
instance Configuration Config where
input = input'
startSt = startSt'
我收到以下错误:
Couldn't match type ‘pin’ with ‘PIn’
‘pin’ is a rigid type variable bound by
the type signature for
input :: (PortIn pin, Show pin) => Config -> pin
at Clks_n_regs_4.hs:101:3
Expected type: Config -> pin
Actual type: Config -> PIn
Relevant bindings include
input :: Config -> pin (bound at Clks_n_regs_4.hs:101:3)
In the expression: input'
In an equation for ‘input’: input = input'
我认为我可以使用 input' ,因为它会在 pin 的实例中产生一个 PIn。
我在某处有误解,希望有人能解释我所缺少的。
最佳答案
要了解为什么您的代码不进行类型检查,请参阅:existential vs. universally quantified types .为了使这项工作,一种可能性是使用 Functional Dependencies .那么你的类型类将是:
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
class (PortIn pin, Show pin, SysState st, Show st) => Configuration c pin st
| c -> pin, c -> st where
input :: c -> pin
startSt :: c -> st
并且实例定义将是:
instance Configuration Config PIn St where
input = input'
startSt = startSt'
这将使您的代码类型检查,尽管可能不是您所追求的。
另一个方向是使用 existential types .在这种情况下:
{-# LANGUAGE ExistentialQuantification #-}
data PortInShow = forall a . (PortIn a, Show a) => PortInShow a
data SysStateShow = forall a . (SysState a, Show a) => SysStateShow a
class Configuration c where
input :: c -> PortInShow
startSt :: c -> SysStateShow
instance Configuration Config where
input = PortInShow . input'
startSt = SysStateShow . startSt'
关于haskell - 在 Haskell 中,如何将类型类与该类型类的实例进行匹配?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/41412958/