[Haskell] 1::Num a => a

Prelude> :t 1
1 :: Num a => a

Prelude> :i Num
class Num a where
  (+) :: a -> a -> a
  (-) :: a -> a -> a
  (*) :: a -> a -> a
  negate :: a -> a
  abs :: a -> a
  signum :: a -> a
  fromInteger :: Integer -> a
   -- Defined in ‘GHC.Num’
instance Num Word -- Defined in ‘GHC.Num’
instance Num Integer -- Defined in ‘GHC.Num’
instance Num Int -- Defined in ‘GHC.Num’
instance Num Float -- Defined in ‘GHC.Float’
instance Num Double -- Defined in ‘GHC.Float’

可见Num是一个type class，而数字1是定义在这个type class中的，具有多态性（Ad Hoc Polymorphism）。
因此，type class中不但可以定义函数，还可以定义值
（当然，函数只是一种具有函数类型的值罢了

例如：

class TypeClassWithValueAndFunc t where
    value :: t
    func :: t -> t

Prelude> :t value
value :: TypeClassWithValueAndFunc t => t

Prelude> :t func
func :: TypeClassWithValueAndFunc t => t -> t

注：
（1）实际上，func只是一个类型为(->) t t的值
（2）从这个角度来讲，type class与OOP中的interface是有不同的，type class不一定是“操作”的集合，而是一些“值”的集合

---

6.4.1 Numeric Literals
The syntax of numeric literals is given in Section 2.5. An integer literal represents the application of the function fromInteger to the appropriate value of type Integer. Similarly, a floating literal stands for an application of fromRational to a value of type Rational (that is, Ratio Integer). Given the typings:

fromInteger :: (Num a) => Integer -> a 
fromRational :: (Fractional a) => Rational -> a

integer and floating literals have the typings (Num a) => a and (Fractional a) => a, respectively. Numeric literals are defined in this indirect way so that they may be interpreted as values of any appropriate numeric type. See Section 4.3.4 for a discussion of overloading ambiguity.

Haskell 2010 Language Report - P82