Num类型类中的歧义

Question

我无法弄清楚为什么在ghci中直接执行代码

Prelude> import Data.Aeson
Prelude Data.Aeson> toJSON 3
Number 3.0

这个简单的程序

import Data.Aeson
main = print . toJSON $ 3

失败并显示错误

/mnt/data/Maggot/tmp.hs:2:16:
    No instance for (ToJSON s0) arising from a use of ‘toJSON’
    In the second argument of ‘(.)’, namely ‘toJSON’
    In the expression: print . toJSON
    In the expression: print . toJSON $ 3
/mnt/data/Maggot/tmp.hs:2:25:
    No instance for (Num s0) arising from the literal ‘3’
    The type variable ‘s0’ is ambiguous
    Note: there are several potential instances:
      instance Num Double -- Defined in ‘GHC.Float’
      instance Num Float -- Defined in ‘GHC.Float’
      instance Integral a => Num (GHC.Real.Ratio a)
        -- Defined in ‘GHC.Real’
      ...plus 17 others
    In the second argument of ‘($)’, namely ‘3’
    In the expression: print . toJSON $ 3
    In an equation for ‘main’: main = print . toJSON $ 3

我知道我可以直接指定type（3 :: Int），但我很好奇如何解决这个问题？ 感谢。