As discussed in the series intro, we are rewriting Stream from Rails to Servant. Since we value small iterations, the idea is to rely as much as possible on existing code while migrating Ruby to Haskell. For that reason, we decided to move endpoint by endpoint and leave the authentication in Rails for the time being.
Stream authenticates users via Slack OAuth. In other words, Rails, for any given authenticated request, knows the slack_token
of the current user. We started with the following endpoint in Servant:
type CommentsAPI =
-- ^ Type definition for the comments Api.
-- ^ For now we just expose one endpoint to create a new comment.
QueryParam "slack_token" Text
-- ^ Expect a query param..
-- ^ ..named `slack_token`..
-- ^ ..and parse it as `Text`.
:> ReqBody '[JSON] CommentRequest
-- ^ The request body..
-- ^ ..will be JSON..
-- ^ ..parsed to a value of type `CommentRequest`.
:> Post '[JSON] Response
-- ^ The endpoint is exposed as a POST..
-- ^ ..it will return a JSON representation..
-- ^ ..of a value of type `Response`.
With the declaration above, the handler function would be something like:
postComment
:: Maybe Text
-> CommentRequest
-> Handler Response
Notice that slack_token
is parsed as Maybe Text
not Text
. In fact, being a query parameter, Servant takes care of the fact that it could be missing.
Since we are passing slack_token
from Rails, we are confident it will always be there. For that reason, we let Servant know so that we do not need to deal with a Maybe
(the docs are pretty clear on the details):
- QueryParam "slack_token" Text
+ QueryParam' '[Required, Strict] "slack_token" Text
postComment
- :: Maybe Text
+ :: Text
-> CommentRequest
-> Handler Response
Let's go one step deeper into the rabbit hole.
Servant parses values to the specified types. For example, we declared that slack_token
will be parsed as Text
. But how does the framework know how to do that? Simple, any value of a type with an instance of FromHttpApiData
can be parsed. Text
, implements it, thus we can use it out of the box.
That means we can ask Servant to parse slack_token
as a value of type SlackToken
:
+ newtype SlackToken = SlackToken Text
+ instance FromHttpApiData SlackToken where
+ parseUrlPiece = Right . SlackToken
- QueryParam' '[Required, Strict] "slack_token" Text
+ QueryParam' '[Required, Strict] "slack_token" SlackToken
postComment
- :: Maybe Text
+ :: SlackToken
-> CommentRequest
-> Handler Response
Now, not only we are not passing an anonymous Text
around, also we could add some validation logic to parseUrlPiece
and return a Left error
in case that failed. In the latter case, Servant would return error
without invoking the handler.