Gotchas and Stack Traces in Ruby
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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.
![#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](https://image.slidesharecdn.com/gotchasandstacktracesinruby-151109192856-lva1-app6892/85/Gotchas-and-Stack-Traces-in-Ruby-6-320.jpg)
Gotchas and Stack Traces in Ruby
- 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.