Mathias Jean Johansen

Common Pitfalls in Ruby

Last week, I stumbled upon a document covering a list of common pitfalls in Ruby that I assembled in the beginning of 2015. Browsing through the various gotchas, I decided to rework the list, and publish it here for future reference. Hopefully, others will find it useful too.

and, or, and not

Since Ruby is often considered a readable language, many novices tend to believe that and, or, and not are more appealing alternatives to &&, ||, and !, respectively. They differ, however, in their behavior. Consider the following example:

foo = true and false
=> false
=> true
foo = true && false
=> false
=> false
foo = false || true
=> true
foo = false or true
=> true
=> false

Similarly, this example below reveals the issue with not versus !.

not true
=> false
=> false
not 3 == 4
=> true
!3 == 4
=> false

In the first example, foo = true and false will be interpreted as (foo = true) and false where foo = true && false will be interpreted as foo = (true && false), as and and or have lower precedence than && and ||. Thus, and and or are meant for flow control, while && and || are for boolean logic.

Correspondingly, not and ! also behave differently due to their precedence. As expected, not has lower precedence than !.

As shown in the second example, not 3 == 4 will be interpreted as not (3 == 4) which evaluates to true, while !3 == 4 will be interpreted as (!3) == 4, i.e. false == 4.

equal?, eql?, ===, and ==

These four methods of determining equality all behave widely different. The first method, equal?, is an identity comparison, so it will only return true in situations where a is the same object as b. One can think of equal? as a pointer comparison.

=> true
=> false

It is worth nothing that if we enable immutable strings in the example above with # frozen_string_literal: true, or the --enable-frozen-string-literal flag, the last example will also evaluate to true.

eql? is essentially for hash comparisons. It returns true when a, and b refer to the same hash key. Hash uses this to test for equality. It is common to alias eql? to ==.

Finally, ===, and == are for case equality, and generic equality, respectively. === is typically overridden to provide meaningful semantics in case statements for Range, Regex, and Proc. == is the most common comparison operator, and therefore this is usually overridden to provide class-specific meaning.

to_s, to_str, and String

to_s, and String are more or less equivalent. String will check the class of its parameter, and if it is not already a string, it will call to_s on it. Calling to_s obviously means it will be called regardless.

to_str is different from the two, however. It should only be implemented in situations where your object acts like a string as opposed to being representable by a string meaning you should only implement to_str in your classes for objects that are interchangeable with String objects.


The Enumerable module in Ruby defines an any? method. When I initially learned Ruby, I expected that it would return true if the collection was non-empty (as a negated empty?). Nevertheless, any? (without a provided block) returns true if at least one of the collection members is not false or nil. The following example demonstrates this behavior:

[false, nil].any?
=> false
[true, false].any?
=> true
[:truthy, nil].any?
=> true

super, and super()

In Ruby, we learn that we can omit parentheses in method calls without any arguments, as foo, and foo() returns the same result, and abandoning unnecessary parentheses is normally what most style guides advocate for.

Consequently, it might be rather tempting to leave out parentheses when calling super() but calling super, and super() is not entirely the same in Ruby. super (without parentheses) will call the parent method with exactly the same arguments that were passed to the original method, while the latter will call the parent method without any arguments at all.

size, count, and length

Similar to other methods in this blog post, people may be tempted to think that size, count, and length are simply aliases for the same operation but this is yet another quirk of Ruby.

length, and size are identical, and they usually run in constant time, so they are faster than count. Unlike count, they are not a part of Enumerable but rather a part of a concrete class (such as Array, or String). Normally, I tend to use length for strings, and size for collections.

As mentioned, count is a part of Enumerable, and it is usually meant to be used with a block, although this is not mandatory.

[1, 2, 3, 4, 5, 6].count(&:even?)
=> 3
[1, 2, 3, 4, 5, 6].count
=> 6[]) vs. {|h, k| h[k] = [] }[]), and {|h,k| h[k] = [] } may look similar but they behave slightly different. When accessing an unknown element,[]) will always return the same array where {|h, k| h[k] = [] } creates a new array. A quick benchmark reveals that accessing an unknown element from a hash initialized with[]) is approximately twice as fast as accessing an unknown element from a hash initialized with {|h,k| h[k] = [] }

This behavior can also be seen in arrays where { } will initialize a new Foo every time, while, will refer to the same Foo object for each element.

The flip-flop operator (..)

In Ruby, .., and ... are most often used for ranges. It allows us to succinctly express ranges from A to Z as such 'a'..'z'. The .. operator always includes the last element where ... will skip the last element in the range.

=> 26
=> 25

We can also conveniently express a date range as such:

require 'date'
=> true
now =
=> #<DateTime: 2016-12-15T22:36:38+01:00 ((2457738j,77798s,446146000n), +3600s, 2299161j)>
last_month = now - 30
=> #<DateTime: 2016-11-15T22:36:38+01:00 ((2457708j,77798s,446146000n),+3600s,2299161j)>
=> 31

The .. operator can, however, lead to a bit of confusion since it has a different behavior in other situations.

(1..20).each do |x|
  puts x if (x == 5) .. (x == 10)

The condition in the loop above evaluates to false every time it is evaluated until the first part, i.e. x == 5, evaluates to true. Then it evaluates to true until the second part evaluates to true. In the above example, the flip-flop is turned on when x == 5 and stays on until x == 10, so the numbers from 5 to 10 are printed.

The flip-flop operator only works inside ifs and ternary conditions. Everywhere else, Ruby considers it to be the range operator. With the flip-flop operator, we now conclude our whirlwind tour of the various pitfalls in the Ruby programming language.

As demonstrated in this blog post, Ruby has quite a few quirks. In order to avoid many of these pitfalls, I usually advocate for using Rubocop either locally on your own machine, or ideally as a part of the CI pipeline. While it may not detect all the issues at hand, it is at most times extremely good at reporting problems in your code.