我正在尝试（未能成功）在运行时创建一个"对象"*，使用依赖类型在运行时定义其类型。
i正在遵循有关依赖类型的教程，并且它们用于在运行时传递值的内容是一个将Sing作为参数的函数，并在Sing的值上使用模式匹配以在运行时获得数字。但是我无法访问任何Sing进行模式匹配。
我认为下面的代码可以工作，但是我得到的实际上令人失望，编译器告诉我，randomNetwork的类型定义中的n与我在SNat的类型定义中捕获的n不同。
{-# LANGUAGE
ScopedTypeVariables, TemplateHaskell, TypeFamilies, GADTs, KindSignatures,
TypeOperators, FlexibleContexts, RankNTypes, UndecidableInstances,
FlexibleInstances, InstanceSigs, DefaultSignatures, DataKinds,
RecordWildCards, StandaloneDeriving, MultiParamTypeClasses #-}
module Main where
-- some imports to make the code below main work
import Control.Monad.Random
import GHC.TypeLits
import Data.List
--import Grenade
import Data.Singletons
import Data.Singletons.TypeLits
main = do
let sizeHidden = toSing (20 :: Integer) :: SomeSing Nat
net0 <- case sizeHidden of
SomeSing (SNat :: Sing n) ->
randomNetwork :: IO (Network '[ FullyConnected 75 n, FullyConnected n 1 ] '[ 'D1 75, 'D1 n, 'D1 1 ])
--net0 <- randomNetwork :: IO (Network '[ FullyConnected 75 3, FullyConnected 3 1 ] '[ 'D1 75, 'D1 3, 'D1 1 ])
print net0
-- from Grenade.Core.Network
data Network :: [*] -> [Shape] -> * where
NNil :: SingI i
=> Network '[] '[i]
(:~>) :: (SingI i, SingI h, Layer x i h)
=> !x
-> !(Network xs (h ': hs))
-> Network (x ': xs) (i ': h ': hs)
infixr 5 :~>
instance Show (Network '[] '[i]) where
show NNil = "NNil"
instance (Show x, Show (Network xs rs)) => Show (Network (x ': xs) (i ': rs)) where
show (x :~> xs) = show x ++ "n~>n" ++ show xs
class CreatableNetwork (xs :: [*]) (ss :: [Shape]) where
randomNetwork :: MonadRandom m => m (Network xs ss)
instance SingI i => CreatableNetwork '[] '[i] where
randomNetwork = return NNil
instance (SingI i, SingI o, Layer x i o, CreatableNetwork xs (o ': rs)) => CreatableNetwork (x ': xs) (i ': o ': rs) where
randomNetwork = (:~>) <$> createRandom <*> randomNetwork
-- from Grenade.Layers.FullyConnected
data FullyConnected i o = FullyConnected
!(FullyConnected' i o) -- Neuron weights
!(FullyConnected' i o) -- Neuron momentum
data FullyConnected' i o = FullyConnected'
!(R o) -- Bias
!(L o i) -- Activations
instance Show (FullyConnected i o) where
show FullyConnected {} = "FullyConnected"
instance (KnownNat i, KnownNat o) => UpdateLayer (FullyConnected i o) where
type Gradient (FullyConnected i o) = (FullyConnected' i o)
runUpdate = undefined
createRandom = undefined
instance (KnownNat i, KnownNat o) => Layer (FullyConnected i o) ('D1 i) ('D1 o) where
type Tape (FullyConnected i o) ('D1 i) ('D1 o) = S ('D1 i)
runForwards = undefined
runBackwards = undefined
-- from Grenade.Core.Layer
class UpdateLayer x where
type Gradient x :: *
runUpdate :: LearningParameters -> x -> Gradient x -> x
createRandom :: MonadRandom m => m x
runUpdates :: LearningParameters -> x -> [Gradient x] -> x
runUpdates rate = foldl' (runUpdate rate)
class UpdateLayer x => Layer x (i :: Shape) (o :: Shape) where
type Tape x i o :: *
runForwards :: x -> S i -> (Tape x i o, S o)
runBackwards :: x -> Tape x i o -> S o -> (Gradient x, S i)
-- from Grenade.Core.Shape
data Shape = D1 Nat
data S (n :: Shape) where
S1D :: ( KnownNat len )
=> R len
-> S ('D1 len)
deriving instance Show (S n)
instance KnownNat a => SingI ('D1 a) where
sing = D1Sing sing
data instance Sing (n :: Shape) where
D1Sing :: Sing a -> Sing ('D1 a)
-- from Grenade.Core.LearningParameters
data LearningParameters = LearningParameters {
learningRate :: Double
, learningMomentum :: Double
, learningRegulariser :: Double
} deriving (Eq, Show)
-- from Numeric.LinearAlgebra.Static
newtype Dim (n :: Nat) t = Dim t
deriving (Show)
newtype R n = R (Dim n [Double])
deriving (Show)
newtype L m n = L (Dim m (Dim n [[Double]]))
如何在运行时定义"隐藏层"的大小（无需手动构造）？我如何在类型级别的运行时使用我捕获的值？
btw，这是我对上述代码遇到的编译错误：
Prelude> :r
net0 <- case sizeHidden of
SomeSing (SNat :: KnownNat n => Sing n) -> randomNetwork :: IO (Network '[ FullyConnected 75 3, FullyConnected 3 1 ] '[ 'D1 75, 'D1 3, 'D1 1 ])
[1 of 1] Compiling Main ( /home/helq/Downloads/NetworkOnRuntime.hs, interpreted )
/home/helq/Downloads/NetworkOnRuntime.hs:23:15: error:
• Couldn't match type ‘a0’
with ‘Network
'[FullyConnected 75 a, FullyConnected a 1] '['D1 75, 'D1 a, 'D1 1]’
because type variable ‘a’ would escape its scope
This (rigid, skolem) type variable is bound by
a pattern with constructor:
SomeSing :: forall k k1 (k2 :: k1) (a :: k). Sing a -> SomeSing k,
in a case alternative
at /home/helq/Downloads/NetworkOnRuntime.hs:22:13-37
Expected type: IO a0
Actual type: IO
(Network
'[FullyConnected 75 a, FullyConnected a 1] '['D1 75, 'D1 a, 'D1 1])
• In the expression:
randomNetwork ::
IO (Network '[FullyConnected 75 n, FullyConnected n 1] '[D1 75,
D1 n,
D1 1])
In a case alternative:
SomeSing (SNat :: Sing n)
-> randomNetwork ::
IO (Network '[FullyConnected 75 n, FullyConnected n 1] '[D1 75,
D1 n,
D1 1])
In a stmt of a 'do' block:
net0 <- case sizeHidden of {
SomeSing (SNat :: Sing n)
-> randomNetwork ::
IO (Network '[FullyConnected 75 n, FullyConnected n 1] '[D1 75,
D1 n,
D1 1]) }
/home/helq/Downloads/NetworkOnRuntime.hs:25:3: error:
• Ambiguous type variable ‘a0’ arising from a use of ‘print’
prevents the constraint ‘(Show a0)’ from being solved.
Relevant bindings include
net0 :: a0 (bound at /home/helq/Downloads/NetworkOnRuntime.hs:21:3)
Probable fix: use a type annotation to specify what ‘a0’ should be.
These potential instances exist:
instance (Show b, Show a) => Show (Either a b)
-- Defined in ‘Data.Either’
instance Show SomeNat -- Defined in ‘GHC.TypeLits’
instance Show SomeSymbol -- Defined in ‘GHC.TypeLits’
...plus 31 others
...plus 170 instances involving out-of-scope types
(use -fprint-potential-instances to see them all)
• In a stmt of a 'do' block: print net0
In the expression:
do { let sizeHidden = ...;
net0 <- case sizeHidden of {
SomeSing (SNat :: Sing n)
-> randomNetwork ::
IO (Network '[FullyConnected 75 n, FullyConnected n 1] '[D1 75,
D1 n,
D1 1]) };
print net0 }
In an equation for ‘main’:
main
= do { let sizeHidden = ...;
net0 <- case sizeHidden of { SomeSing (SNat :: Sing n) -> ... };
print net0 }
Failed, modules loaded: none.
*：我知道，我们称它们为价值（我认为）
让我们考虑一下这一行：
net0 <- case sizeHidden of
SomeSing (SNat :: Sing n) ->
randomNetwork :: IO (Network '[ FullyConnected 75 n, FullyConnected n 1 ] '[ 'D1 75, 'D1 n, 'D1 1 ])
net0的类型是什么？它似乎是
Network '[ FullyConnected 75 n, FullyConnected n 1 ] '[ 'D1 75, 'D1 n, 'D1 1 ]
但是，什么是n？它不在环境中，因为main的类型环境是空的。而且它也不是普遍量化的。这就是问题所在，n无需参考任何内容。您需要在n绑定 1 的环境中使用net0进行所有工作，如
case sizeHidden of
SomeSing (SNat :: Sing n) -> do
net0 <- randomNetwork :: IO (Network '[ FullyConnected 75 n, FullyConnected n 1 ] '[ 'D1 75, 'D1 n, 'D1 1 ])
print net0
或将net0包裹在其自身的存在中：
data DogeNet =
forall n. KnownNat n => DogeNet (Network '[ FullyConnected 75 n, FullyConnected n 1 ]
'[ 'D1 75, 'D1 n, 'D1 1 ])
instance Show DogeNet where -- deriving can't handle existentials
show (DogeNet n) = show n
main = do
...
net0 <- case sizeHidden of
SomeSing (SNat :: Sing n) ->
DogeNet <$> (randomNetwork
:: IO (Network '[ FullyConnected 75 n, FullyConnected n 1 ]
'[ 'D1 75, 'D1 n, 'D1 1 ]))
print net0
randomNetwork仍然需要类型签名，因为我们需要指出我们真正打算在上一行上使用绑定的n，从而迫使我们两次编写网络规格。但是可以使用新的TypeApplications扩展名清理：
DogeNet @n <$> randomNetwork
1 这不会使事情看起来像它看起来那么不可能。您仍然可以将net0传递给n中普遍量化的功能。这只是意味着，如果您返回涉及 new 类型级别编号的类型，则需要通过CPS进行或使用DogeNet。