我已经找到了一种方法,可以使用Nat
和Integer
将Proxy
转换为natVal
,如下面的代码所示:
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Main where
import Data.Proxy (Proxy)
import Data.Monoid ((<>))
import GHC.TypeLits
main :: IO ()
main = do
fromNat (undefined :: Proxy 5)
fromNat :: KnownNat n => Proxy n -> IO ()
fromNat proxy = do
let (num :: Integer) = natVal proxy -- converting a Nat to an Integer
putStrLn $ "Some num: " <> show num
但是我无法想出一种将类型列表转换为常规列表的简单方法,下面的代码甚至不会进行类型检查:
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Main where
import Data.Proxy (Proxy)
import Data.Monoid ((<>))
import GHC.TypeLits
main :: IO ()
main = do
fromNat (undefined :: Proxy 5)
fromListNat (undefined :: Proxy '[2,3,10])
fromNat :: KnownNat n => Proxy n -> IO ()
fromNat proxy = do
let (num :: Integer) = natVal proxy -- converting a Nat to an Integer
putStrLn $ "Some num: " <> show num
fromListNat :: Proxy [Nat] -> IO ()
fromListNat = undefined
如何将类型列表转换为常规列表?
答案 0 :(得分:5)
答案是制作类似KnownNat
的内容,但适用于Nat
的类型级别列表。我们使用类型类在类型级别列表上进行归纳。这个类型类通过其超类约束,将检查列表中的所有元素是否满足KnownNat
,然后它将使用该事实来重构术语级列表。
{-# LANGUAGE TypeOperators, KindSignatures #-}
-- Similar to `KnownNat (n :: Nat)`
class KnownNatList (ns :: [Nat]) where
natListVal :: proxy ns -> [Integer]
-- Base case
instance KnownNatList '[] where
natListVal _ = []
-- Inductive step
instance (KnownNat n, KnownNatList ns) => KnownNatList (n ': ns) where
natListVal _ = natVal (Proxy :: Proxy n) : natListVal (Proxy :: Proxy ns)
然后,fromListNat
采用与fromNat
相同的形状:
fromListNat :: KnownNatList ns => Proxy ns -> IO ()
fromListNat proxy = do
let (listNum :: [Integer]) = natListVal proxy
putStrLn $ "Some list of num: " <> show listNum
将这些更改拼接到您的初始代码中,我得到了预期的输出:
$ ghc Main.hs
$ ./Main
Some num: 5
Some list of num: [2,3,10]