failing gracefully

Scala provides a util package which provides a handy Try type, with its companions Success and Failure.

This reminds me very much of standard promise handling in JavaScript browser apps.

Here’s a little code to try an http call (this one is destined to fail). When it fails, the “failure” will throw a nasty exception java.io.FileNotFoundException with stack trace. We can catch that error and return it as an instance of Failure, to be unwrapped later by some code which is ready for it.

import scala.util.{Try, Success, Failure}

def fetch(url: String) = scala.io.Source.fromURL(url).mkString

val url = "https://httpbin.org/hidden-basic-auth/user/passwd"

val data = Try(fetch(url)) match {
  case Success(value) => Success(value)
  case Failure(error) => Failure(error)
}

println(data)

From the terminal:

scala exception.scala
# => Failure(java.io.FileNotFoundException: https://httpbin.org/hidden-basic-auth/user/passwd)

And the Failure class has lots of useful methods to unwrap the data.

Not saying we should always swallow stack traces…usually a bad idea. But in some cases, it is helpful to know that an error may be coming and to handle it gracefully.