Reading Ruby Error Messages

Learning Goals

Read the different parts of an error message
Identify common types of errors

Introduction

In this lab, you'll be reading error messages from tests. This lab is designed so that both running the files and running the test suite via the learn test command will show the error messages for you to decode. Moving forward though, you'll be reading error messages mainly through running the test suite.

Reading Error Messages

Let's start by running some of the Ruby code in the lib folder to produce an error message. Run this in your terminal:

$ ruby lib/a_name_error.rb

Error messages have 3 parts:

lib/a_name_error.rb:3:in `<main>': undefined local variable or method `hello_world' for main:Object (NameError)

The location of the error, the "where":
```
lib/a_name_error.rb:3:in `<main>':
```
- lib/a_name_error.rb is the file the error occurred in.
- 3 is the line of code with the error.
- <main> is the scope of the error.
The description, the "why":
```
undefined local variable or method `hello_world' for main:Object
```
The interpreter does the best job it can to tell you what it thinks went wrong.
The type of error, the "who":
```
(NameError)
```
This is a Ruby Error Type.

You've solved games of Clue with less information. This is one of the best parts of programming: debugging and fixing errors. It's like you're a detective solving a crime. The only bad thing is that more often than not, you're also the criminal that caused the error in the first place.

Errors are clues, and reading them is the interpreter telling you what to do to fix the program and move on.

Four Common Error Types

Name Errors

Name errors are caused when a given name is invalid or undefined. Whenever the Ruby interpreter encounters a word it doesn't recognize, it assumes that word is the name of a variable or a method. If that word was never defined as either a variable or a method, it will result in a name error. Try this out in IRB:

$  irb
2.7.3 :001 > a_variable
NameError (undefined local variable or method `a_variable' for main:Object)
2.7.3 :002 > a_variable = 7
 => 7
2.7.3 :003 > a_variable
 => 7

Trying to access a_variable before assigning it a value results in a NameError, which we can fix by assigning it some value.

Syntax Errors

Syntax errors are pretty self-explanatory: they're the result of incorrect syntax. Thankfully, they're usually followed by a guess about the location of the error. For example, imagine we had included the following code inside some Ruby code:

2.times do
  puts "hi"

Running our code would result in:

2: syntax error, unexpected end-of-input, expecting keyword_end

Here, Ruby is saying that on line 2, there is a missing end (every do keyword must be followed by some code and then an end keyword). Always read the full details of syntax errors and look for line numbers, which usually appear at the beginning of the error message.

Note: You won't be able to reproduce the above syntax error using IRB, because IRB won't execute the code until you enter a matching end keyword for the do block. You can see this error by creating a Ruby file, adding the code above, and executing the file.

Type Errors

When you try and do a mathematical operation on two objects of a different type, you will receive a TypeError. For example if you try and add a string to an integer, Ruby will complain.

1 + "1"

Will produce the following error:

TypeError: String can't be coerced into Fixnum

Division Errors

A DivisionError is caused when a given number is divided by 0.

Instructions

To get started, run learn test --f-f to run the first test in the test suite. Use the error messages to guide your work:

Read the errors. Scroll through the entire output to get a sense of what the failures are trying to tell you. What does the error mean? How can we fix it?
Each error prints out a stack trace, which points to where the code failed and attempts to follow it up the stack — that is, through the bits of code that ran leading up to the failure. You can use these stack traces to pinpoint which line(s) of code need your attention.
These stack traces can also point you to which files you should run to get a better sense of the errors.

Fix the errors in each of the files in lib/. Then confirm the fix by running learn test again.

winstonkiprop/phase-3-reading-error-messages