沒有用於（Convertible Int Double）的實例

Question

任何人都可以解釋此錯誤消息嗎？根據Hackage，Data.Convertible.Base，版本1.0.11.1的實例聲明爲Convertible Int Double。

λ> :set -package=convertible-1.0.11.1 
package flags have changed, resetting and loading new packages...                  
λ> import Data.Convertible.Base 
λ> let b = 3 :: Int 
λ> convert b :: Double 

<interactive>:18:1: 
    No instance for (Convertible Int Double) 
     arising from a use of `convert' 
    Possible fix: 
     add an instance declaration for (Convertible Int Double) 
    In the expression: convert b :: Double 
    In an equation for `it': it = convert b :: Double 

Answer 1

Haddock文檔顯示包中定義的實例。這並不一定意味着它們與數據類型在相同的模塊中定義。你應該

import Data.Convertible 

或至少

import Data.Convertible.Instances