Haskell：多个文件的酸状态？

Question

我有这样的文件结构：

--- Database.hs ---
data Database = ...
...

--- User.hs ---
import Database
addUser :: Update Database ()
...

--- Post.hs
import Database
addPost :: Update Database ()
...

问题是我需要在makeAcidic ''Database [...]和User.hs中调用Post.hs以使其功能变为酸性，但makeAcidic同时生成IsAcidic Database的实例所以GHC抱怨重复的实例声明。

Answer 1

好的，我明白了。只需将定义与Template Haskell分开：

--- Database/Types.hs ---
data Database = ...
...

--- Database.hs ---
import Database.Types
import User
import Post
makeAcidic ''Database [addUser, addPost]

--- User.hs ---
import Database.Types
addUser :: Update Database ()
...

--- Post.hs ---
import Database.Types
addPost :: Update Database ()
...