實例聲明哈斯克爾

Question

我的代碼是這樣的：

type Code = [Inst] 

data Inst = PUSH Int 
      | PUSHV Name 
      | POP Name 
      | DO Op 
      | JUMP Label 
      | JUMPZ Label 
      | LABEL Label 
      deriving Show 

doJump :: Inst -> Code -> Code 
doJump l c = case (elemIndex l c) of 
       Just n -> drop (n+1) c 
       Nothing -> c 

而且GHC返回我這個錯誤，這讓我完全難倒... 我試圖做的是從後點返回列表發生特定元素。

No instance for (Eq Inst) arising from a use of `elemIndex' 
Possible fix: add an instance declaration for (Eq Inst) 
In the expression: (elemIndex l c) 
In the expression: 
    case (elemIndex l c) of { 
    Just n -> drop (n + 1) c 
    Nothing -> c } 
In an equation for `doJump': 
    doJump l c 
     = case (elemIndex l c) of { 
      Just n -> drop (n + 1) c 
      Nothing -> c } 

任何想法？

Answer 1

elemIndex要求列表中的元素是相等的，因此您需要爲Eq Inst添加實例。

使用deriving Eq，系統將自動生成的，例如：

data Inst = PUSH Int 
      | PUSHV Name 
      | POP Name 
      | DO Op 
      | JUMP Label 
      | JUMPZ Label 
      | LABEL Label 
      deriving (Show, Eq)