Graceful Exceptions

When creating a HTTP request we never know if the targeted resource is available. Let’s look at the following example in Kotlin. Using khttp the following passes

val res = get("http://hodler.co")
assertThat(res.statusCode).isEqualTo(200)

Boring. We force an error

val res = get("http://notavailablehodler.co")
assertThat(res.statusCode).isEqualTo(200)

Leads to

Exception in thread “main” java.net.UnknownHostException: notavailablehodler.co

How did that happen?

Apparently get was supposed to return type Response

It did not.

We got UnknownHostException

How would we know, without reading its implementation or the documentation, the function might throw UnkownHostException?

Yes, through experience we might guess it can fail. But not whether it will raise an Exception or return null.

Thus, if we already know something could fail shouldn’t we somehow add it to the function declaration?

Using Arrow, a functional companion to Kotlin’s standard library, we create the following abstraction

fun fetchInfos(): Either<String, Response> {
    try {
        return Right(get("http://hodler.co"))
    } catch (e: Exception) {
        return Left("Something went wrong")
    }
}

Either

The Either is a well known concept in functional programming.

It’s used to signal whether an error can occur.

While Right, by convention, is used to hold the values of a successful execution, Left is used to show something went wrong. Usually in the form of an error message.

As a result the caller of the function will know from it’s declaration whether a function might return an error.

It’s not hidden from plain sight. We’re forced to deal with it.

Usage of the function created above might look as follows

fetchInfos()
  .fold({ "Service currently not available" },
    { "Service has returned statuscode $it.status" })

The fold allows us to extract the value from Either. Or provide a default if it’s Left.

Say we use the above to render a message to the user. Should fetchInfos return successfully by providing Right then the user will see

Service has returned statuscode 200

On the other hand providing a Left will show

Service currently not available

Guess all we would have to do now is to create a HTTP request library which will only return the type Either instead of Response or some kind of exception.