Feb 13, 2017 6:00:53 AM | Ruby Exception Handling: CompatibilityError

A in-depth examination of Ruby's CompatibilityError, which appears when attempts are made to compare two values which have incompatible encoding types.

As we continue to make our way through the expansive Ruby Exception Handling series, today we'll be taking a closer look at the CompatibilityError. CompatibilityErrors appear when dealing with encoding in Ruby, and specifically when attempts are made to compare two encoded values which have incompatible encoding types.

Throughout this article we'll delve into the details of the CompatibilityError class, examining where it sits within Ruby's Exception class hierarchy, as well as how to handle any CompatibilityErrors that you may deal with personally. Let's get going!

The Technical Rundown

  • All Ruby exceptions are descendants of the Exception class, or a subclass therein.
  • StandardError is a direct descendant of the Exception class, and is also a superclass with many descendants of its own.
  • EncodingError is a direct descendant of the StandardError class, and is also a superclass with a handful of descendants of its own.
  • CompatibilityError is a direct descendant of the EncodingError class.

When Should You Use It?

Character encoding within Ruby (or even in general development) can be a bit confusing to say the least. Often it's a bit of a "black box" affair, where methods are called to perform encoding and conversions without any clear understanding of how Ruby is handling things.

Thankfully, as newer Ruby versions have been released and steady improvements have been included with each, the challenges of working with encoding have certainly lessened, but headaches still abound when a small snippet of code refuses to function because one tiny little character isn't compatible with some encoding check deep down in your codebase.

For this very reason, Ruby includes a handful of built-in encoding exception classes, the first of which is the CompatibilityError we're examining today.

As mentioned in the introduction, the CompatibilityError will appear anytime two strings with incompatible encodings are compared in some way. To understand what this means, let's first look at a working example:

# UTF-8 instance
utf8 = "hello"
# Convert to ASCII
forced = "hello".encode('ASCII')
# Compare the two
puts utf8.include? forced

Here we're creating a new string with the text "hello". By default, my character encoding in Ruby is UTF-8, so all newly generated strings are automatically assigned UFT-8 encoding, hence our variable name of utf8.

We're also creating a second instance of our "hello" string, encoding it to ASCII using the encode() method, and assigning the value to our forced variable.

Finally, we compare the two by checking to see if one value (utf8) includes the other value (forced), and outputting the true/false result:


As expected, there are no problems, and even though the encodings of UTF-8 and ASCII for both strings differ, the byte-representations of the characters that make up the simple string of "hello" are equivalent in both encodings, thus the comparison works fine. That is, the ASCII decimal value of the lowercase letter h is 104, and the UTF-8/Unicode decimal value for h is also 104, so comparing these two simple strings is no problem.

However, what happens if we change our string to use characters which are not represented in the limited ASCII character set? For example, let's try the word résumé:

def print_exception(exception, explicit)
puts "[#{explicit ? 'EXPLICIT' : 'INEXPLICIT'}] #{exception.class}: #{exception.message}"
puts exception.backtrace.join("\n")

# UTF-8 instance
utf8 = "résumé"
# Convert to ASCII
forced = "résumé".force_encoding('ASCII')
# Output the encoding of forced
puts forced.encoding
# Compare the two
puts utf8.include? forced
rescue Encoding::CompatibilityError => e
print_exception(e, true)
rescue => e
print_exception(e, false)

We've added some helper functions to handle potential exceptions. We've also changed our call to the encode method to force_encoding, because a call to encode fails since our string contains values not found in the ASCII encoding set. Lastly, we're also outputting the encoding value of our forced variable, just to double-check that the forced encoding actually took.

What's important to understand here is that calling the force_encoding() method doesn't actually change the characters (and thus the underlying string) in anyway; it's simply a hack of sorts to inform Ruby of what we consider to be the "correct" encoding for this string. Ruby will then use that encoding type value for later execution, as we'll see right now.

The end result of all this is that we're asking Ruby to compare our two strings (which are both identical characters of "résumé") via the include?() method, but one of them has been forced to use ASCII encoding. Since ASCII has no representation for the handful of special characters we are using, this comparison fails and raises the expected CompatibilityError:

[EXPLICIT] Encoding::CompatibilityError: incompatible character encodings: UTF-8 and US-ASCII
code.rb:34:in `include?'
code.rb:34:in `<main>'

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Written By: Frances Banks