Haskell中如何弃用特定的类型类实例？

可以实现，你遇到的解析错误是因为DEPRECATED编译指令的放置位置不符合Haskell语法规范。

Haskell的编译pragma（如DEPRECATED）属于顶层声明，不能放在缩进的实例方法定义块内部。正确的做法是将DEPRECATED pragma放在要标记的实体上方：

正确代码示例 #

{-# DEPRECATED fromRow @Job "Please do not depend on the FromRow instance of the Job type. It does not handle optional features, such as job-results and job-workflows. Depending upon your scenario, use 'jobRowParserSimple' or 'jobRowParserWithWorkflow' directly." #-}
instance FromRow Job where
  fromRow = jobRowParserSimple

补充说明 #

1. 如果不需要精确限定到Job类型的fromRow，也可以简化为：

{-# DEPRECATED fromRow "Please do not depend on the FromRow instance of the Job type. It does not handle optional features, such as job-results and job-workflows. Depending upon your scenario, use 'jobRowParserSimple' or 'jobRowParserWithWorkflow' directly." #-}
instance FromRow Job where
  fromRow = jobRowParserSimple

不过添加@Job类型限定能更精准地标记目标，避免影响其他类型的fromRow方法。

2. 若要标记整个FromRow Job实例而非单个方法，可这样写：

{-# DEPRECATED instance FromRow Job "Please do not depend on the FromRow instance of the Job type. Use 'jobRowParserSimple' or 'jobRowParserWithWorkflow' directly instead." #-}
instance FromRow Job where
  fromRow = jobRowParserSimple

内容的提问来源于stack exchange，提问作者Saurabh Nanda