Rewriting to Haskell–Parsing Query Params

Posted on April 27, 2020 by Riccardo.

Rewriting to Haskell (Series)

Rewriting to Haskell–Intro Rewriting to Haskell–Project Setup Rewriting to Haskell–Deployment Rewriting to Haskell–Automatic Formatting Rewriting to Haskell–Configuration Rewriting to Haskell–Standing on the shoulders of Rails Rewriting to Haskell–Making GHC More Nitpicky Rewriting to Haskell–Testing Rewriting to Haskell–Linting Rewriting to Haskell–Parsing Query Params Rewriting to Haskell–Parsing Query Params, Again Rewriting to Haskell–Errors

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..
--          ^ will return a JSON representation..
--                  ^ ..of a value of type `Response`.

With the declaration above, the handler function would be something like:

  :: 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

-   :: 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

-   :: 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.

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