How to validate parameters in a functional way?

Question

I'm writing Scala/Play 2.0 application, and I want my code to return different page depending on request. Here is my code: ``` // Validate client and return temporary credentials def requestToken = Action { request => // Authorization header may present or not val authHeader = parseHeaders(request headers AUTHORIZATION) // Authorization header may contain such keys or not val clientKey = authHeader("oauth_consumer_key") val signature = authHeader("oauth_signature") if (authenticateClient(clientKey, signature)) { ... Ok(...) } else { Unauthorized(...) } ``` } The problem arise when request is malformed and some headers are missing, then NoSuchElementException is thrown. In imperative language, I would validate every step like this: ``` if (!request.headers.contains(AUTHORIZATION)) return Unathorized val authHeader = parseHeaders(request headers AUTHORIZATION) if (!authHeader.contains("oauth_consumer_key") || !authHeader.contains("oauth_signature")) return Unathorized val clientKey = au…

Accepted Answer

You can use `authHeader.get(key)` which returns an Option[B]. Your code then would look something like this:

```
val result = for {
  auth <- request.headers.get(AUTHORIZATION)
  authHeader = parseHeaders(auth)
  clientKey <- authHeader.get("oauth_consumer_key")
  signature <- authHeader.get("oauth_signature")
} yield { ... }

result.getOrElse(Unauthorized)
```

Explanation:

The whole expression will only be executed, if there is no `None` value. So if all headers are present you will get a `result` of `Some[A]` and if one or more are absent, you will get `None`. `result.getOrElse(Unauthorized)` will return the contained value for `Some[A]` and `Unauthorized` for `None`.

How to validate parameters in a functional way?

Solution

Problem