使用stable-aarch64-apple-darwin编译Netrium代码遇Semigroup推导错误求助

错误原因 #

新版GHC编译器中，Monoid类的定义要求其类型必须先属于Semigroup类（Semigroup是Monoid的超类），你提供的代码仅定义了Monoid (TimedEvents a)实例，未提前定义对应的Semigroup实例，导致编译器无法自动推导约束条件。

解决方法 #

两种简单修改方式任选其一即可：

方法1：先定义Semigroup实例（推荐）

在Monoid实例代码之前，添加Semigroup实例定义，复用你已有的合并逻辑（Semigroup核心操作<>和mappend语义一致）：

instance Semigroup a => Semigroup (TimedEvents a) where
  (<>) = mappend

同时将原Monoid实例的上下文保留为Monoid a即可（Monoid本身已包含Semigroup约束）：

instance Monoid a => Monoid (TimedEvents a) where
  mempty        = TEs []
  mappend as bs =
      fmap mappendMergeResult (mergeEvents as bs)
    where
      mappendMergeResult (OnlyInLeft  a)   = a
      mappendMergeResult (InBoth      a b) = a `mappend` b
      mappendMergeResult (OnlyInRight   b) =             b

方法2：直接修改Monoid实例的上下文约束

在Monoid实例的上下文里添加Semigroup (TimedEvents a)约束，明确告知编译器该类型满足Semigroup要求：

instance (Monoid a, Semigroup (TimedEvents a)) => Monoid (TimedEvents a) where
  mempty        = TEs []
  mappend as bs =
      fmap mappendMergeResult (mergeEvents as bs)
    where
      mappendMergeResult (OnlyInLeft  a)   = a
      mappendMergeResult (InBoth      a b) = a `mappend` b
      mappendMergeResult (OnlyInRight   b) =             b

操作步骤 #

1. 找到项目中的src/Observations.hs文件
2. 按照上述任一方法修改对应代码片段
3. 重新执行编译命令即可

内容的提问来源于stack exchange，提问作者mac