Haskell: acid-state over multiple files?

Question

I have a file structure like this:

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

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

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

The problem is that I need to called makeAcidic ''Database [...] in both User.hs and Post.hs to make their functions acidic, but makeAcidic at the same time generate IsAcidic instances for Database so GHC complains about duplicate instance declarations.

Answer 1

OK, so I figure it out. Just separate the definition with the Template Haskell:

--- 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 ()
...