问如何创建一个映射usign、GADT和Data.Map的类型化玩具语言环境？

EN

{-# LANGUAGE GADTs, RankNTypes #-}

module Foo where

import qualified Data.Map as M
import Data.Maybe

data Expr a where
  V :: Char -> Expr a
  B :: Bool -> Expr Bool
  I :: Int -> Expr Int
  If :: Expr Bool -> Expr a -> Expr a -> Expr a

type Env a = M.Map Char (Expr a)

env = M.fromList [('x', B True), ('y', I 10)]
-- Error here                      ----^^^^
-- [typecheck -Wdeferred-type-errors] [E] • Couldn't match type ‘Int’ with ‘Bool’
--   Expected type: Expr Bool
--     Actual type: Expr Int
-- • In the expression: I 10
--   In the expression: ('y', I 10)
--   In the first argument of ‘M.fromList’, namely
--     ‘[('x', B True), ('y', I 10)]’