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tl;dr: There's an update to Persistent's type generation which greatly simplifies error messages at the expense of losing some not-often-needed generality. You can turn this generality on or off with a simple boolean flag, so if you want the full power, it's available. For most users, I recommend turning it off as described below. As an example of error message simplification, compare:

Actual type: GHandler sub0 master0 (Maybe (Entity (UserGeneric Database.Persist.GenericSql.Raw.SqlBackend))) -> (Maybe (Entity (UserGeneric Database.Persist.GenericSql.Raw.SqlBackend)) -> GHandler sub0 master0 (Maybe (Key (UserGeneric Database.Persist.GenericSql.Raw.SqlBackend)))) -> GHandler sub0 master0 (Maybe (Key (UserGeneric Database.Persist.GenericSql.Raw.SqlBackend)))

Using the new simplified types, this turns into:

Actual type: GHandler sub0 master0 (Maybe (Entity User)) -> (Maybe (Entity User) -> GHandler sub0 master0 (Maybe (Key User))) -> GHandler sub0 master0 (Maybe (Key User))

The problem: multi-backend datatypes

(If you just want to make your life easier and don't care about the details, feel free to skip to the next section.)

Persistent works with multiple database backends. One issue that comes into play for this is textual serialization of database keys. In the case of our SQL backends, we have simple numerical keys. So any piece of text that entirely consists of digits could be a valid key. On the other hand, MongoDB has a much more specific key format.

The solution to this was to parameterize database keys on the backend they applied to, in addition to the data type they apply to. In other words, we have:

data Key backend entity

You can see more about this motivation in an email I wrote over a year ago. Now suppose you want to embed a reference to a different entity in a current one, e.g.:

Person name Text Car owner PersonId make Text model Text

PersonId gets converted to Key backend Person ... but how do we know what backend is? Up until now, Persistent's solution to this is to create a separate CarGeneric datatype which is parameterized on backend , and to create a helper Car synonym that uses the backend specified in your MkPersistSettings . In other words, you get:

data PersonGeneric backend = Person Text type Person = PersonGeneric SqlBackend data CarGeneric backend = Car (Key backend (PersonGeneric backend)) Text Text type Car = CarGeneric SqlBackend

This lets you keep some generic data types in case you want to store your data in multiple backends (like PostgreSQL and MongoDB). However, it results in much more complicated error messages. And given that this is such a corner use case, this solution was- in retrospect- a bad tradeoff.

So now, if you specify mpsGeneric as False (or use sqlOnlySettings ), the generic datatypes won't be created. Instead, you'd just get:

data Person = Person Text data Car = Car (Key SqlBackend Person) Text Text

If you're only using a single database backend, this should introduce no downsides.

Start using the update

You basically need to make two changes to your site to use this new feature:

Depend on persistent-template 1.1.1 or greater. In Model.hs , change sqlSettings to sqlOnlySettings (or the equivalent for Mongo).

Note that you'll need to be using persistent 1.1.

If you're a library author creating reusable components (e.g., an admin interface), you should stick with the generic datatypes to avoid tying down to a single backend. But for most standard applications, I highly recommend the change.

Future work

I'm hoping this is just one step of many for simplifying error messages in the Yesod ecosystem. We've done a lot of that in the past, but I have some new ideas. If you want to have a look at a concrete idea, you can look at an experimental typeclass-based conduit library, based around an idea I saw from Chris Smith a while ago.

I'm hoping to write separate blog posts about thoughts on conduit and classy-prelude, but I'll save that for another time.