This document discusses common "gotchas" and issues when working with Ruby, along with explanations of stack traces. It covers: 1. Syntax errors due to whitespace when calling methods. 2. The behavior of the "any?" method on arrays and how it checks for "truthy" elements. 3. That constants in Ruby can be reassigned with a warning, and examples of variable scope errors. 4. Examples of stack traces for different errors and the importance of including the full trace when asking for help to identify the root cause.

1. Gotchas and Stack
Traces in Ruby
OmbuLabs, September 2015

2. #1: Whitespace
def method(arg1, arg2)
end
method 1, 2 # OK
method(1, 2) # OK
method (1, 2) # Error!

3. SyntaxError: (irb):24: syntax error, unexpected ',',
expecting ‘)'
from (irb):7:in `method'

4. #1: Whitespace
def method
42
end
num = 21
method/num # OK
method / num # OK
method/ num # OK
method /num # Error!

5. ArgumentError: wrong number of arguments (1 for
0)
from (irb):7:in `method'

6. #2: True & False
[].any? => false
[1].any? => true
[:foo, :bar].any => true
[false].any? => false
[nil].any? => false
[false, nil].any? => false
[false, true].any? => true

7. `any?` is actually “Are any elements truthy?”

8. #3: Constants (not)
FOO = 5
=> 5
FOO = 7
(irb):3: warning: already initialized constant FOO
=> 7
FOO
=> 7

9. Constants can be reassigned, Ruby
will just throw a warning and let you
do it anyway.

10. def foo
0/0
end
def bar
foo
end
def baz
bar
end
baz

11. ➜ ombushop git:(develop) ✗ ruby test.rb
test.rb:2:in `/': divided by 0 (ZeroDivisionError)
from test.rb:2:in `foo'
from test.rb:6:in `bar'
from test.rb:10:in `baz'
from test.rb:13:in `<main>'

12. def foo
my_correctly_spelled_variable = "hey!"
puts my_incorrectly_spelled_variable
end
def bar
foo
end
def baz
bar
end
baz

13. ➜ ombushop git:(develop) ✗ ruby test.rb
test.rb:3:in `foo': undefined local variable or method
`my_incorrectly_spelled_variable' for main:Object (NameError)
from test.rb:7:in `bar'
from test.rb:11:in `baz'
from test.rb:14:in `<main>'

14. def foo
my_correctly_spelled_variable = "hey!"
puts my_incorrectly_spelled_variable
end
def bar
my_incorrectly_spelled_variable = "hey!"
foo
end
def baz
my_incorrectly_spelled_variable = "hey!"
bar
end
baz

15. undefined local variable or method
`my_incorrectly_spelled_variable' for main:Object
(NameError)

16. ➜ ombushop git:(master) ✗ ruby test.rb
test.rb:3:in `foo': undefined local variable or method
`my_incorrectly_spelled_variable' for main:Object (NameError)
from test.rb:8:in `bar'
from test.rb:13:in `baz'
from test.rb:16:in `<main>'

20. TL;DR: Always paste the full stack
trace of the error you come across
when asking for help.

21. THANK YOU!
questions?