Question

I have recently learned the Ruby programming language, and all in all it is a good language. But I was quite surprised to see that it was not as simple as I had expected. More precisely, the ‘rule of least-surprise’ did not seem very respected to me (of course this is quite subjective). For example:

x = true and false puts x  # displays true! 

and the famous:

puts 'zero is true!' if 0  # zero is true! 

What are the other ‘Gotchas’ you would warn a Ruby newbie about?

Answer 1

Wikipedia Ruby gotchas

From the article:

• Names which begin with a capital letter are treated as constants, so local variables should begin with a lowercase letter.
• The characters $ and @ do not indicate variable data type as in Perl, but rather function as scope resolution operators.
• To denote floating point numbers, one must follow with a zero digit (99.0) or an explicit conversion (99.to_f). It is insufficient to append a dot (99.), because numbers are susceptible to method syntax.
• Boolean evaluation of non-boolean data is strict: 0, '' and [] are all evaluated to true. In C, the expression 0 ? 1 : 0 evaluates to 0 (i.e. false). In Ruby, however, it yields 1, as all numbers evaluate to true; only nil and false evaluate to false. A corollary to this rule is that Ruby methods by convention — for example, regular-expression searches — return numbers, strings, lists, or other non-false values on success, but nil on failure (e.g., mismatch). This convention is also used in Smalltalk, where only the special objects true and false can be used in a boolean expression.
• Versions prior to 1.9 lack a character data type (compare to C, which provides type char for characters). This may cause surprises when slicing strings: 'abc'[0] yields 97 (an integer, representing the ASCII code of the first character in the string); to obtain 'a' use 'abc'[0,1] (a substring of length 1) or 'abc'[0].chr.

• The notation statement until expression, unlike other languages’ equivalent statements (e.g. do { statement } while (not(expression)); in C/C++/…), actually never runs the statement if the expression is already true. This is because statement until expression is actually syntactic sugar over

  until expression   statement end 

  , the equivalent of which in C/C++ is while (not(expression)) statement; just like statement if expression is an equivalent to

  if expression   statement end 

  However, the notation

  begin   statement end until expression 

  in Ruby will in fact run the statement once even if the expression is already true.

• Because constants are references to objects, changing what a constant refers to generates a warning, but modifying the object itself does not. For example, Greeting << ' world!' if Greeting == 'Hello' does not generate an error or warning. This is similar to final variables in Java, but Ruby does also have the functionality to ‘freeze’ an object, unlike Java.

Some features which differ notably from other languages:

• The usual operators for conditional expressions, and and or, do not follow the normal rules of precedence: and does not bind tighter than or. Ruby also has expression operators || and && which work as expected.

• def inside def doesn’t do what a Python programmer might expect:

  def a_method     x = 7     def print_x; puts x end     print_x end 

  This gives an error about x not being defined. You need to use a Proc.

Language features

• Omission of parentheses around method arguments may lead to unexpected results if the methods take multiple parameters. The Ruby developers have stated that omission of parentheses on multi-parameter methods may be disallowed in future Ruby versions; the current (November 2007) Ruby interpreter throws a warning which encourages the writer not to omit (), to avoid ambiguous meaning of code. Not using () is still common practice, and can be especially nice to use Ruby as a human readable domain-specific programming language itself, along with the method called method_missing().