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Question

关于SYB，我还是个初学者。

我试着编写一段代码来获得表达式中的变量

 variables = removeDuplicate $ (everything (++)  ([] `mkQ` f))
          where
             f (ExprVar st) = [st]
             f _  = []

其中removeDuplicate消除了列表中的重复变量

ExprVar是我的数据类型Datatype还包括用于加法、减法乘法、除法和数字的ExprAdd、Exprsub、ExprMul、ExprDiv、ExprNum。

我在编译时遇到以下错误：

No instance for (Data a0) arising from a use of `everything'
    The type variable `a0' is ambiguous
    Possible cause: the monomorphism restriction applied to the following:
      variables :: a0 -> [[Char]] (bound at ParserExpr.hs:107:1)
    Probable fix: give these definition(s) an explicit type signature
                  or use -XNoMonomorphismRestriction
    Note: there are several potential instances:
      instance Data Expr -- Defined at ParserExpr.hs:24:28
      instance (Data a, Data b) => Data (a -> b)
        -- Defined in `Data.Generics.Instances'
      instance Data DataType -- Defined in `Data.Generics.Instances'
      ...plus 43 others
    In the expression: (everything (++) ([] `mkQ` f))
    In an equation for `variables':
        variables
          = (everything (++) ([] `mkQ` f))
          where
              f (ExprVar st) = [st]
              f _ = []

请让我知道我哪里错了？

谢谢

Answer 1

错误消息准确地指出了问题所在，并提供了两种可能的解决方案，这两种方案都有效：

在GHC中添加Data a => a -> [String] as type signature

• enable -XNoMonomorphismRestriction标志(或添加LANGUAGE杂注)

有关更多信息，请参阅https://www.haskell.org/haskellwiki/Monomorphism_restriction