typeclass实例内的typeclass约束

你好,在做来自现实世界哈斯克尔书的例子时,我遇到了这个例子,我无法理解它的含义和工作原理:
instance Num a=>Num (SymbolicManip a)
在这种情况下,我应该将其转换为类似这样的内容:“例如num instance of type SymbolicManip 对于其类型的字段有一个约束 a ,即: 一 是num本身的实例吗? 有人能告诉我我是正确解释还是解释?
为什么不呢 instance Num (SymbolicManip a) 够了吗?

-- The "operators" that we're going to support
data Op = Plus | Minus | Mul | Div | Pow
        deriving (Eq, Show)

{- The core symbolic manipulation type -}
data SymbolicManip a = 
          Number a           -- Simple number, such as 5
        | Arith Op (SymbolicManip a) (SymbolicManip a)
          deriving (Eq, Show)

{- SymbolicManip will be an instance of Num.  Define how the Num
operations are handled over a SymbolicManip.  This will implement things
like (+) for SymbolicManip. -}
instance Num a => Num (SymbolicManip a) where
    a + b = Arith Plus a b
    a - b = Arith Minus a b
    a * b = Arith Mul a b
    negate a = Arith Mul (Number (-1)) a
    abs a = error "abs is unimplemented"
    signum _ = error "signum is unimplemented"
    fromInteger i = Number (fromInteger i)

附笔 所有代码都是从书上写的( 章 13 分章 -扩展示例数值类型)