鉴于下面的代码在Data.HashMap中查找类型的类型特定信息，是否可以定义注释中记录的新函数getMapVal2来构建{{1}给出类型的参数？
TypeKey这只是一个玩具代码，用于在运行时通过{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DataKinds #-}
import Data.Monoid ((<>))
import Data.Proxy (Proxy(Proxy))
import GHC.TypeLits (KnownSymbol, Symbol, symbolVal)
import qualified Data.HashMap.Strict as Map (HashMap, empty, insert, lookup)
import Data.Dynamic
import GHC.Generics
import Data.Maybe (fromJust, isNothing, maybe)
type family TypeKey (a :: *) :: Symbol where
TypeKey Int = "int"
TypeKey T = "trec"
data T = T { aInt :: Int} deriving (Show, Generic, Typeable)
extract ::(s ~ TypeKey a, Typeable a, KnownSymbol s) => Maybe Dynamic -> Maybe a
extract dyn = if (isNothing dyn) then Nothing else fromDynamic . fromJust $ dyn
getMapVal :: (s ~ TypeKey a, Typeable a, KnownSymbol s) => Map.HashMap String Dynamic -> String -> Maybe a
getMapVal m k = extract $ Map.lookup k m
{-- How do we get the TypeKey lookup for type a?
getMapVal2 :: (s ~ TypeKey a, Typeable a, KnownSymbol s) => Map.HashMap String Dynamic -> a -> Maybe a
getMapVal2 m ty = extract $ Map.lookup (symbolVal (Proxy :: Proxy (TypeKey ???))) m
--}
main = do
let map = Map.insert (symbolVal (Proxy :: Proxy (TypeKey T))) (toDyn $ T {aInt=5}) Map.empty -- we insert some value in hashmap for type T - it is of same type
val = getMapVal map (symbolVal (Proxy :: Proxy (TypeKey T))) :: Maybe T -- now let us retrieve the value in map for Type T. We pass the SymbolVal ourselves
--val = getMapVal2 map (T {aInt = 2}) -- now we want to lookup map value given something of a type T. Need getMapVal2 to build symbolval given the input type
print $ maybe "" show val -- prints value stored in Hashmap for type T which is: T {aInt=5}
测试传递类型特定的配置到作用于类型类型的多态函数。
答案 0 :(得分：4)
使用the ScopedTypeVariables extension。这允许您在绑定它们的定义的主体中引用forall - 绑定类型变量。
{-# LANGUAGE ScopedTypeVariables #-}
getMapVal2 :: forall a s. (s ~ TypeKey a, Typeable a, KnownSymbol s) => Map.HashMap String Dynamic -> a -> Maybe a
getMapVal2 m ty = extract $ Map.lookup (symbolVal (Proxy :: Proxy (TypeKey a))) m