snaplet-rest

REST resources for the Snap web framework.

REST resources for the Snap framework.

As an example, let's translate the following datatype into a resource.

data User = User Username String Int

type Username = CI String

We need a type to represent changes to the resource. This partial type indicates what properties to change: either the name, the age, or both.

data UserPart = UserPart (Maybe String) (Maybe Int)

This type also acts as a search mechanism: we can search by names, ages, or both. We can use either a username or a UserPart search to find users, and define a function to convert URL query string parameters to this search.

type UserId = Either Username UserPart

userIdFromParams :: Params -> Maybe UserId

Now we have the pieces required to define our CRUD behaviour.

createUser :: User -> AppHandler ()

readUser :: UserId -> AppHandler [User]

updateUser :: UserId -> UserPart -> AppHandler Bool

deleteUser :: UserId -> AppHandler Bool

If we've implemented Aeson instances, we can add JSON as a media format without having to define these manually. Once the behaviour is attached to the resource, it can be served in the handler.

serveUser :: AppHandler ()
serveUser = serveResource $ resource
    & addMedia jsonInstances
    & setCreate createUser
    & setRead readUser
    & setUpdate updateUser
    & setDelete deleteUser
    & setFromParams userIdFromParams