haskell - Haskell TypeSynonymInstances 实例声明

Question

尝试使用 ghci> 加载文件时出现以下错误：l myfile.hs

• Illegal instance declaration for ‘ExpSYM String’

    (All instance types must be of the form (T t1 ... tn)

     where T is not a synonym.

     Use TypeSynonymInstances if you want to disable this.)

• In the instance declaration for ‘ExpSYM String’

在行中：实例 ExpSYM String where

---

class ExpSYM repr where
    lit :: Int -> repr
    neg :: repr -> repr
    add :: repr -> repr -> repr
    
instance ExpSYM Int where
    lit n = n
    neg e = - e
    add e1 e2 = e1 + e2
    
eval :: Int -> Int
eval = id



instance ExpSYM String where
    lit n = show n
    neg e = "(−" ++ e ++ ")"
    add e1 e2 = "(" ++ e1 ++ " + " ++ e2 ++ ")"

view :: String -> String
view = id

---

起初我以为这是因为双实例 ExpSYM，但如果我删除第一个（实例 ExpSYM Int）并且它是 eval，我仍然会看到错误。怎么修？问题是什么？

标签： haskell