Ribbit

Ribbit is yet another type safe relational database library for Haskell, heavily inspired by the amazing Servant library. The goal is to create a type-level language for defining table schemas "as a type", queries that operate on those schemas, and, tangentially, "backends" that can do something useful with those types like talk to an actual database.

Using Ribbit, you might expect to see something like this:

type PeopleTable =
  Field "id" Int
  :> Field "name" Text
  :> Field "age" Int
  

type MyQuery = Select '["id", "name"] `From` PeopleTable `Where` "age" `Equals` (?)

matchingPeople <-
  query
    dbConn
    (Proxy :: Proxy MyQuery)
    (Only 21) -- argument that fills in the (?) placeholder

  :: IO [Only Int :> Only Text]

Status

The status of Ribbit is non-functional pre-alpha. My goal though is to make sure it is absolutely production ready for the operations it ends up supporting, but we are a long way from that at the moment.

How it compares with other libraries.

The short answer is there are a lot of other libraries and I'm not sure. Persistent and esquelleto are ones I've used, but if you search "relational" or "sql" in Hackage there seems to be a lot of other options. Part of the goals for this library are to flesh out this approach myself, so I can have a better context for understanding everything else available. In other words, it is part research project. With that in mind, there are at least a couple of specific goals I have in mind:

• Avoid template Haskell. Persistent is amazing, but the use of Template Haskell makes certain things difficult, like documenting (or for large projects even understanding) everything that is produced by the Template Haskell.

• Make the language easy to understand. If you have some basic SQL knowledge, it should be immediately obvious what is going on even if you are a beginner Haskeller.

• Try to make as much stuff happen at the type level as possible. The ability to write your own type classes or type families over Servant API types is, I feel, part of what makes Servant so amazing. I want to replicate that success here. So, for instance, if someone somewhere defines a schema type that looks like this:

  type MySchema =
    Field "id" Int
    :> Field "name" Text
    :> Field "address" (Maybe Text)

  Then you would be free to deconstruct this type (using type families), transform it into another schema, generate customized CREATE TABLE statements if the (forthcoming) ones provided aren't good enough for your back-end or use case... that sort of thing. As a somewhat contrived example, maybe, for who knows what reason, you never want to allow null values in your database. You can write a type family that can inspect every field in an arbitrary schema, replacing all the Maybe a with just a, like:

  -- With -XPolyKinds
  type family NoNulls schema where
    NoNulls (Field name (Maybe typ)) = Field name typ
    NoNulls (a :> b) = NoNulls a :> NoNulls b
    NoNulls a = a
  
  NoNulls MySchema 
  -- Same as:
  --   Field "id" Int
  --   :> Field "name" Text
  --   :> Field "address" Text <--- note the lack of Maybe

The name: Ribbit

The name means nothing except I kindof like the sound of it. There are so many "sql", "relational", "query", etc. package names already that I didn't want to:

1. get lost in the mix.
2. step on anyone's toes by choosing too similar a name.
3. create confusion by seeming to be associated with some other package with which I am not.